Add config file support

This change adds two depdencies - mixlib-cli and mixlib-config. Strictly
speaking only mixlib-config is needed for config file support, but I wanted to
switch from optparser to mixlib-cl anyway and the two libraries place nice
together. Currently anything you can specify on the command line can also be
specified in the config file. The config file defaults to ~/.mdlrc, but you
can change it with -c.

Refs #13
Move main program logic into lib/mdl.rb

This change also puts the main program logic inside the MarkdownLint module,
removing the MarkdownLint::Config stuff. I also split cli.rb into its own
file.

Fixes: #13