Working with locally modified plugins

[lieryan]

The scenario is that I am making a number of local modifications to plugins managed by minpac. There are two cases:

1. The repository may have dirty/uncommitted files in the working directory
2. The local and remote branch have branched away from each other

Currently, in both cases, :call minpac#update() will fail because minpac complained that it cannot do a fast forward merge.

Proposal: add an option to specify that a plugin should be updated with git pull --rebase --autostash or git pull --rebase instead of the current behaviour which is git pull --ff-only.

If pull rebase caused a merge conflict, minpac should issue git rebase --abort and then warn the user that some plugins failed to update and that they have to resolve the conflict manually.

[k-takata]

I implemented this in the 'devel' branch in #101.