Add left-stripping functionality

[adityasaky]

in-toto's python and go implementations provide an option to provide patterns that can be used to left-strip artifact paths. One example where this is important is #12, where instead of recording /tmp/rebuilderd7oil7Z/inputs/vultr-cli-2.8.3-1-x86_64.pkg.tar.zst, we want to record vultr-cli-2.8.3-1-x86_64.pkg.tar.zst.

For reference, the python implementation can be found here: https://github.com/in-toto/in-toto/blob/develop/in_toto/runlib.py#L94
And the go implementation can be found here: https://github.com/in-toto/in-toto-golang/blob/master/in_toto/runlib.go#L179