Load user-wide setting file ~/.erdconfig

It overrides default options.
But execution-specific options (like `rake erd attributes=content`) override it.
added RailsERD::ConfigFile to load config and test it
Make configuration file comma-removable

Now we don't need to write comma `:` for keys and values.
See examples/erdconfig.example.
Make RailsERD::ConfigFile#load to load $PWD/.erdconfig

RailsERD::ConfigFile#load loads $PWD/.erdconfig if exists.
If both $PWD/.erdconfig and $HOME/.erdconfig exists, it loads
$HOME/.erdconfig first, and override it with $PWD/.erdconfig.
Add installation of graphviz to before_install
Merge pull request #90 from kerrizor/add-graphviz-to-travis

CI: Add installation of graphviz to before_install
Merge pull request #36 from a2ikm/erdconfig

Load user-wide setting file ~/.erdconfig