Add the blame keyword for improved findability

[mssever]

This closes issue #37

As described there:

I don't often have a need for a tool like Annotator, but when I do, it's really useful. However, due to the time gap between invocations, I'm unlikely to remember that it uses the word "annotate" in the command palette. The usual term I use (as used by several VCS programs, Github, etc.) is "blame." If I'm searching for the tool, I'm likely to type blame and be unable to find this plugin.

I would love to be able to find this plugin's features by typing blame.