Use -r more than once to specify multiple files

Use the `-r` parameter multiple times to specify multiple requirements files.

I had problems using the comma separated list. This failed:

`caniusepython3 -r ~/a.txt,~/b.txt`

...because the shell won't interpret the ~ after the comma. This also failed:

`caniusepython3 -r ../a.txt,../b.txt`

...for the same reason.

If you'd prefer not to integrate this change, then perhaps do path interpretation on the requirement file paths. Using multiple `-r`'s feels more UNIXy to me, but I don't care *that* much.

Thanks for this very cool utility, by the way. :cool: