This consolidates code across tools in `spack style` so that each
`run_<tool>` function can be called indirecty through a dictionary
of handlers, and os that checks like finding the executable for the
tool can be shared across commands.
- [x] rework `spack style` to use decorators to register tools
- [x] define tool order in one place in `spack style`
- [x] fix python 2/3 issues to Get `isort` checks working
- [x] make isort error regex more robust across versions
- [x] remove unused output option
- [x] change vestigial `TRAVIS_BRANCH` to `GITHUB_BASE_REF`
- [x] update completion
24a4d81097