Make it possible to override gap_element

[reynoldsnlp]

'-' as a gap element doesn't work for my project.

Currently, I am doing the following, but it seems pretty hack-ish:

import alignment
alignment.vocabulary.GAP_ELEMENT = '∅'

This pull request would make it possible to define these values when initializing the Vocabulary, like this Vocabulary(gap_element='∅').

I don't understand why GAP_ELEMENT is defined in a different file. Maybe you don't need the constants at all, and just have defaults in the __init__().