Add option to merge equal feature names in BED flatten script.

This pull request adds new flag in flatten.py (-m) to not concatenate feature names that are equal when flattening BED files.

Basically, with the flag switched to true, instead of feature names like this:

GENEA, GENEB, GENEB, GENEC

We'll only have this

GENEA, GENEB, GENEC

I left the default flag value to be false, so it doesn't change the previous script behavior.

Let me know if more changes need to be made :).