Move dictionary name to optional parameter

Most often users just need the default dictionary, so let's create an option
with default value set to dictonary.txt. Users can overwrite this parameter with
its own dictionary.

By default codespell will check $PWD/data/dictionary.txt file. It is where
dictionary is stored if one uses codespell from sources tar or git repo.
It is expected that package managers will install dictionary to shared data folder
and change default value accordinally.