Thanks should support multiple ways to specify packages to check

[phildini]

From discussion in #pypa on freenode:

It would be great if thanks <package name> just worked, even on packages that weren't installed locally.

[tomdottom]

@phildini were you thinking of following the pip cli api.

thanks -r requirements.txt

thanks package-name

thanks setup.py

???

[phildini]

So, here's my vision for the interface. All of these are listed in "order in which I would like to see them implemented"

1. thanks with no arguments - looks at the results of pip list, fetches the funding data for everything it finds.
2. thanks -r requirements.txt - looks only at the requirements.txt, fetches the funding data for what's listed there
3. thanks {package_name} - fetches the funding data of a single package.

Note, when I say "fetches" here, I mean "should check locally first and then try to optionally hit PyPI"

Deep future:

4. thanks -p Pipfile - does what thanks -r requirements.txt does, but on a Pipfile
5. thanks -s setup.py - same, but on a setup.py