style: simplify arguments with --tool TOOL and --skip TOOL

`spack style` tests were annoyingly brittle because we could not easily be
specific about which tools to run (we had to use `--no-black`, `--no-isort`,
`--no-flake8`, and `--no-mypy`). We should be able to specify what to run OR
what to skip.

Now you can run, e.g.:

    spack style --tool black,flake8

or:

    spack style --skip black,isort

- [x] Remove  `--no-black`, `--no-isort`, `--no-flake8`, and `--no-mypy` args.
- [x] Add `--tool TOOL` argument.
- [x] Add `--skip TOOL` argument.
- [x] Allow either `--tool black --tool flake8` or `--tool black,flake8` syntax.

share/spack/spack-completion.bash

@@ -1697,7 +1697,7 @@ _spack_stage() {
 _spack_style() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -b --base -a --all -r --root-relative -U --no-untracked -f --fix --no-isort --no-flake8 --no-mypy --no-black --root"
+        SPACK_COMPREPLY="-h --help -b --base -a --all -r --root-relative -U --no-untracked -f --fix --root -t --tool -s --skip"
     else
         SPACK_COMPREPLY=""
     fi