Add `:CorpusNew`

[wincent]

Since 887e3f9 I often create new files by doing :Corpus foo bar.md.

But if the strings foo and bar.md happen to exist in a note file, hitting enter will just open that file; there is not way to signal your intent to create a new file.

So, let's do one of the following:

• Add :CorpusNew ... to cover this use case.
• Or :Corpus! ....
• Or teach :Corpus to grok options, so you could type, for example, :Corpus foo bar --new or --create or whatever).