Add safeguards against duplicate paths on the rtp

There was a bug where if you tried to maktaba#plugin#Install a plugin that was
already on the runtimepath as a RELATIVE path, then maktaba would also add the
ABSOLUTE version of the plugin path to the runtimepath. This would allow the
plugin to be sourced twice.

Fixing this involved adding a maktaba#rtp#Remove function.
Bump version to 1.1.0

Added the maktaba#rtp#Remove function.
Regenerated help docs.

Also, tweaked the wording in maktaba#rtp#Join() which was somewhat unclear.
Make rtp#Add and rtp#Remove respect the rtp cache

The cache was only being updated by rtp#Split.
typo fix: he -> the
Move rtp.vim constants outside of the resource-guard.
Merge pull request #15 from google/docs

Regen docs (maktaba#string changes)
Fix bug in rtp.vim causing undefined s:cache_string
Merge pull request #17 from google/rtp

Fix bug in rtp.vim causing undefined s:cache_string
Typos I noticed in vroom/pluginfiles and later
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.
Merge pull request #21 from chiphogg/typos_i_noticed

Typos I noticed in vroom/pluginfiles and later
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$'
Fix typo "spaces" -> "underscores"

Was startled there for a few seconds.
Merge pull request #24 from glts/fix-canonical-name-typo

Fix typo "whitespace" -> "underscores"