Support environment markers 

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" :)