Scope Vim functions so they can be mapped elsewhere.
Make Vim mailbox check keys available in any buffer.
Add configuration of python binary path for mailbox scripts.

Also replaces subprocess.check_output calls with Popen() +
communicate(), so the plugin works Python <2.7.
Make g:diffscuss_config optional for Vim.
Update Vim configuration docs in README.
Add some basic in-Vim docs for the Vim plugin.
Add some basic implementation notes for the Vim plugin.
Fix a regexp in depth().

The depth calculation uses a regexp that previously counted mixed header
and body symbols (e.g. "%--*--" == depth 5). It now correctly counts the
first run of contiguous symbols (e.g. "%--*--" == depth 2).
Fix edge case with editor.find_header_start.
Start adding some basic unit tests for editor.