Support environment markers

[facundobatista]

Recently a "fades -r requirements.txt" failed because in the file there was something like:

pysha3==1.0b1; python_version < '3.6'

A quick search showed two PEPs that may be relevant:

https://www.python.org/dev/peps/pep-0496/
https://www.python.org/dev/peps/pep-0508/

Fades should "do the right thing" :)