acai-cli: add --ref flag

[MartinHelmut]

Like already mentioned in #2 there should be a flag to pass in the reference / range of commit(s) --ref / -r. This could be used like:

$ acai --ref "v1.0.0..v2.0.0"

to calculate spots based on the range of version 1 to 2. A ref could also be a single commit (or a commit range):

$ acai --ref "4f7abfd"

To calculate the spots from the (included) commit 4f7abfd to current HEAD.

Remark: This feature is also compatible with Mercurial (#4)