[cli] add input type specifier to generate content command

[nilslice]

As referenced in the community suggestion project, this PR adds the ability to define which type of HTML input to show in an editor view for a content type in the CMS.

The specifier is entirely optional, and if not added the behavior is exactly the same as prior versions. To use, add a third parameter to the generated field, like so:

$ ponzu gen c song title:string artist:string:input description:string:richtext genre:string:select

In the content/song.go file generated, the fields' views will automatically be the following:
Title: Input
Artist: Input
Description: Richtext
Genre: Select

This saves time in the set-up phase for your content types and should also be helpful if you're looking for documentation on how to use each editor input func from the editor package. The following input view types are implemented:

CLI parameter	Generates
checkbox	editor.Checkbox()
custom	generates a pre-styled empty div to fill with HTML
file	editor.File()
hidden	editor.Input() + uses type=hidden
input, text	editor.Input()
richtext	editor.Richtext()
select	editor.Select()
textarea	editor.Textarea()
tags	editor.Tags()