Permit configuration extensions

[alexeagle]

Created from a commit by @b0inbot :

An example which allows configuring internal details of the formatter. Replaces the switch statement in format.sh with a JSON that is handled. (dont love depending on jq here).

Obviously a lot more is needed for this to be accepted: doc work, porting over mirror_linguist_languages.sh, a format_config or format_extensions function within defs.bzl.

Testing was done by running bazel run format_custom within the test folder and seeing the .bozo file getting processed by the mock yaml formatter.

penne[130][test] $ pwd
/home/boin/Projects/tools/tools/rules_lint/format/test
penne[000][test] $ bazel run format_custom
INFO: Analyzed target //format/test:format_custom (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //format/test:format_custom up-to-date:
  bazel-bin/format/test/format_custom.bash
INFO: Elapsed time: 0.093s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/format/test/format_custom.bash
+ yamlfmt example/hello.bozo
Formatted YAML in 0m0.022s