Spell-check Markdown by type, not file extension

Previously, only `*.md` files would get spell-checking. This change adds
`*.markdown` and decouples Markdown spell-checking by file extension.

http://robots.thoughtbot.com/vim-spell-checking/
Disable Rubygem documentation to make Bundler fast
Add Git credentials.
Initialise rbenv on startup.
Add Homebrew directory to PATH.
Move personal details to local files.
Add Github Enterprise settings.
Add Github CLI.
Look for GitHub Enterprise settings.