make it possible to run pre and post commands

[marten-seemann]

Running the same command in every directory is a bit limiting. For example, when creating git commits, what we actually want to do is collect files from all directories (git add), and then just git commit once.
This happened here for example: ipfs/go-datastore#179.

Proposed API:

steps:
  - name: Code Quality Checks
    uses: protocol/multiple-go-modules@master
    with:
      working-directory: scripts
      pre: echo "I'm only run once, in the parent directory"
      run: touch file && git add file
      post: git commit -m "add a file per directory"

@mvdan, what do you think?

[mvdan]

Why not do those pre/post steps as... regular job steps, before and after the multiple-go-modules action step? :)