Add fake (empty) plugins with special-cased names

"dash-it-off.vim" should become "dash_it_off".
"vim-unimpaired" should become "unimpaired".
Vroom test for special-cased canonical names
Canonical names: drop '^vim-' and '\.vim$'

These conventions are common on github, but keeping them in the canonical name
is just noise.
Compare resolved paths in #GetOrInstall()

Fixes #18.
Keep call to s:Fullpath()

And be sure to use the paths with trailing slash in the error message.
Merge pull request #23 from glts/resolve-plugin-paths

Compare resolved paths in #GetOrInstall()
Merge pull request #22 from chiphogg/drop_the_vim

Canonical names: disregard '^vim-' and '\.vim$'