.alignment() should return values in a consistent format

[chrislit]

For the new ._soft_src_tar_int() member of TokenDistance to work correctly, it expects .alignment to return in a format like Levenshtein.alignment does:
(value, src-aligned, tar-aligned)
Here, value is the dist_abs value and the two aligned strings have '-' in place of insertions/deletions. Other classes with .alignment() members should return (at least optionally) in the same format.

[chrislit]

Completed at 5112dbc.
There's still perhaps the question of how 'standard' ALINE's format is, since it uses pipes & extra spaces. Whether to address this depends on how useful ALINE is when using soft intersections.