Make ain respect VISUAL first and EDITOR second

So apparently $EDITOR is second in command
and $VISUAL is first. Aincent stuff, but
let's do what most programs do and
respect the first over the second.

Test plan:
* Have $EDITOR set to
  vim. export VISUAL=code --wait
  and run ain template.ain!.
  Verify code opens and waits
  for code to close.
* Unset VISUAL and export
  EDITOR=code --wait. Run
  ain with the same template and
  verify code opens and waits
  for editing.
* Unset both VISUAL and EDITOR
  and verify vim opens as the default
  with the test-case above.

Author: jonaslu

README.md

@@ -209,14 +209,15 @@ Anything after a pound sign (#) is a comment and will be ignored.
 
 Ain accepts one or more template-file(s) as a mandatory parameter. As sections combine or overwrite where it makes sense you can better organize API-calls into hierarchical structures with increasing specificity. An example would be setting the [[Headers]](#Headers), [[Backend]](#backend) and [[BackendOptions]](#BackendOptions) in a base template file and then specifying the specific [[Host]](#Host), [[Method]](#Method) and [[Body]](#Body) in several template files, one for each API-endpoint. You can even use an `alias` for things you will always set.
 
-Adding an exclamation-mark (!) at the end of the template file name makes ain open the file in your `$EDITOR` (or vim if not set) so you can edit the template file. Any changes are not stored back into the template file and used only this invocation.
+Adding an exclamation-mark (!) at the end of the template file name makes ain open the file in your `$VISUAL` or `$EDITOR` or defaults to vim editor in that order so you can edit the template file. Any changes are not stored back into the template file and used only this invocation.
 
 Example:
 ```
 ain templates/get-blog-post.ain!
 ```
 
-Ain waits for the `$EDITOR` command to exit. Any terminal editor such as vim, emacs, nano etc will be fine. If your editor of choice forks (such as [vscode](https://code.visualstudio.com/) does by default) check if there's a flag stopping it from forking. For example to stop vscode from forking use the `--wait` [flag](https://code.visualstudio.com/docs/editor/command-line#_core-cli-options):
+Ain waits for the editor command to exit. Any terminal editor such as vim, emacs, nano etc will be fine. If your editor of choice forks (such as [vscode](https://code.visualstudio.com/) does by default) check if there's a flag stopping it from forking. For example to stop vscode from forking use the `--wait` [flag](https://code.visualstudio.com/docs/editor/command-line#_core-cli-options):
+
 ```
 export EDITOR="code --wait"
 ```

internal/pkg/disk/read.go

@@ -14,14 +14,22 @@ import (
 const editFileSuffix = "!"
 
 func captureEditorOutput(tempFile *os.File) (string, error) {
-	editorEnvStr := os.Getenv("EDITOR")
+	editorEnvVarName := "VISUAL"
+	editorEnvStr := os.Getenv(editorEnvVarName)
+
+	if editorEnvStr == "" {
+		editorEnvVarName = "EDITOR"
+		editorEnvStr = os.Getenv(editorEnvVarName)
+	}
+
 	if editorEnvStr == "" {
+		editorEnvVarName = "vim"
 		editorEnvStr = "vim"
 	}
 
 	editorCmdAndArgs, err := utils.TokenizeLine(editorEnvStr)
 	if err != nil {
-		return "", errors.Wrap(err, "Cannot parse EDITOR environment variable")
+		return "", errors.Wrapf(err, "Cannot parse $%s environment variable", editorEnvVarName)
 	}
 
 	editorArgs := append(editorCmdAndArgs[1:], tempFile.Name())
@@ -38,7 +46,7 @@ func captureEditorOutput(tempFile *os.File) (string, error) {
 
 	err = cmd.Run()
 	if err != nil {
-		return "", errors.Wrapf(err, "Error running $EDITOR %s", cmd.String())
+		return "", errors.Wrapf(err, "Error running $%s %s", editorEnvVarName, cmd.String())
 	}
 
 	_, err = tempFile.Seek(0, 0)