This PR tried to reduce the aegir node_modules weight but failed completely :(

On the bright side all deps are up to date, some logic was simplified and direct deps number was reduced.

Contributors feature now uses git-authors-cli which is faster, orders contributors, dedupes authors and removes bots.

BREAKING CHANGE: hooks no longer support callbacks, eslint packages were updated so errors maybe appear and contributors list feature will change a bit.

Generalises the `test-external` command to replace arbitrary modules
in third party repos and run their tests.

Run it like this:

```console
$ aegir test-dependant git@github.com:org/project.git --deps=foo@1.0.0,bar@2.0.0
```

`git@github.com:org/project.git` will be cloned to a temporary folder,
dependencies installed and tests run.  Then the dependencies specified
in the `--deps` arg will be upgraded and the tests run again.

`--deps` should contain the project you want to test, and can be anything npm
can resolve as a version number, for example github URLs:
`--deps=foo@github:org/project.git#$TRAVIS_COMMIT`

Lerna based monorepos are supported.

Also splits commit/tagging into two separate commands and checks
for file modifications before committing updated contributor lists.