The `Editor` is very useful for correctly opening a text editor and making changes
to some initial input. The current use of `Editor` is only for getting input for
commets.
However, it would also be useful for opening up a text editor on some other
existing files or text, for example, in the command `rad config edit`.
The `Editor` struct was changed to have two new options, `truncate` and
`cleanup`, to allow the user of the struct to dictate whether existing text is
truncated, and if the underlying file should be remove.
The original `new` method is now named `comment`, mimicing the existing
construction of a temporary `RAD_COMMENT` file.
The new version of `new` accepts any file for the `Editor` and will open it
without truncating or removing the file.
`Editor` is now used for the `rad config edit` command.
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