Add Insert mode command and <Plug> keybind again

[w0rp]

An early test version of this plugin had an Insert mode command. We should add it back again, with the following additions.

1. Running the command should make it easy to enter the prompt without "leaving" Insert mode.
2. We should modify Neural to insert text where the user's cursor is in Insert mode, no matter where it moves to.
3. We should detect the user leaving Insert mode and automatically cancel Neural's text input if the user does.

If we set it up this way, it will be very easy to just add some text to a Vim buffer. If we run into issues with entering text into command, we could make it so you type characters and press enter whilst in insert mode, and we replace that text with the results of the prompt. We also have the option of implementing insert mode prompting that way anyway if it turns out to be nice.