Add support for modes.

By default, filepirate acts as previously. However, if
g:filepirate_is_modal=1, then filepirate starts in "insert mode" but
will allow you to press ESC to enter "normal mode", in which most of the
File Pirate mappings are removed and standard Vim mappings can be used
(e.g. for searching and yanking). Corresponding customisable mappings
were added.