User-defined contexts

[dbarnett]

We should figure out how to let users create their own contexts without building and installing a modified version of abominade (possibly without even writing code in some cases?).

Some tools use special paths relative to some arbitrary root directory, and in cases like that it doesn't work properly to click on paths to edit them in vim since the file lookup fails. It would be nice to be able to extend LocalContext to recognize special paths.

Also may make sense to define custom contexts with custom actions.