Add support for config file

If present, lstap will now consult ~/.lstaprc for a list of bar aliases. Right
now, only one section is recognized, called [bars], which contains key-value
pairs of ``<alias>: <url>''. For example, my config file looks like:

   mauchter@think lstap % cat ~/.lstaprc
   [bars]
   draughthouse: http://www.taplister.com/bars/40b13b00f964a5209af31ee3
   hopfields: http://www.taplister.com/bars/4e1a2449aeb7eb0120dbf401
   mauchter@think lstap %

Then, instead of providing the url as the argument, you can substitute an alias.
   mauchter@think lstap % lstap --date --verbose draughthouse | tail -1
   [2013-01-27] Southern Star Brewing Co. - Southern Star Rauchbier