Help people find "Uninstall Docs"

[markstos]

trafficstars

Helps address #121. Before, if you searched the readme for "uninstall", there were no results. Which is odd, because "Uninstall" seems like a natural opposite to "Install".

Also, "Removing directories" did not clearly communicate that the directories being removed were plugin directories.

As #121 showed, clearly a lot of people were challenged to figure out how to uninstall based on the current docs.

I think it would also be good to add the "Uninstall" keyword to the Tutorial wiki page, but the wiki doesn't appear to allow open edits, so I can't make that change myself: https://github.com/junegunn/vim-plug/wiki/tutorial#removing-plugins

[blueyed]

CI failure should be unrelated - flaky? (https://travis-ci.org/junegunn/vim-plug/jobs/445184808)

[markstos]

This patch has been rebased so it can merge cleanly.