Avoid unnecessary rtp remove/re-add in some cases

Fixes #49.
Correct #Install docs on addon-info.json case for plugin name
Fix maktaba doc header to look right in ":help local-additions"
Fix uncaught E716 by explicitly checking has_key(…, 'name')

Somehow vim ends up not catching the E716 in maktaba#plugin#Install and
letting it bubble up.
Merge pull request #53 from google/help

Fix help doc header to look right in ":help local-additions".
Merge pull request #54 from google/addon_name

Fix uncaught E716 by explicitly checking has_key(…, 'name')
Bump version to 1.2.1
Merge pull request #52 from google/safe_rtp

Avoid unnecessary rtp remove/re-add in some cases
Merge branch 'master' into syscall