feature request: local usage

It would be nice to be able to use some features of this package locally, on one's own computer, *on demand*.

Eg I could imagine something like
```julia
julia> import CompatHelper

julia> CompatHelper.show_possible_upgrades("~/code/to/my/repo")
# just list information, eg Foo could be upgraded 0.1.2 => 0.3
# do not modify files

julia> CompatHelper.propose_possible_upgrades("~/code/to/my/repo")
# when there are no uncommitted changes, modify files, otherwise abort
```
The user could then review and test these.

Looking at the sources, I think a lot of the building blocks for this are actually available.