Add "addons" to pattern of non-plugin dir names

Fixes #51.
Don't sanitize official plugin names, only g:installed_{plugin}

Resolves #55.
Inline function call for g:installed_{PLUGIN}
Merge pull request #56 from google/detect_addons

Add "addons" to pattern of non-plugin dir names
Merge branch 'master' of github.com:google/maktaba into no_sanitize
Bump version to 1.2.2
Merge pull request #57 from google/no_sanitize

Don't sanitize official plugin names, only g:installed_{plugin}
Merge branch 'master' into syscall
Bump version to 1.3.0