Add usage mode to support not resolving dependencies

[micahjsmith]

I am conceptually trying to use liccheck in a CI job as follows:

pipenv lock -r > requirements.txt
liccheck -r requirements.txt

However this fails due to #42 because liccheck tries to resolve all dependencies but they have not actually been installed in any virtualenv. It isn't necessary to resolve dependencies here because the requirements file was generated from the lockfile and has pinned dependencies that have already been resolved. Installing a bunch of dependencies is not desired for what should be a lightweight CI job.

Is it possible to add a usage mode to support this, or does liccheck require all the dependencies to be installed?

[atugushev]

Hey @micahjsmith,

I've added --no-deps in #48. Any feedback/test/review is welcomed.