Add PryTheme::Config and define .paint_key_as_symbol!

Closes #30 (Colours of Ruby 1.9 Symbols are off)

You have to invoke `PryTheme::Config.paint_key_as_symbol!` inside your
'.pryrc'.
Refactor Config and test it

So from now on you should use this:

  Pry.config.theme_options = {:paint_key_as_symbol => true}

theme_options must be a hash, because in case we need to add any other
options, it will be easy to add them.
CHANGELOG.md: add information about the new feature