use .buildifier-tables.json file without specifying on CLI

[peakschris]

We need to use a custom tables.json file with --add_tables or --tables. But it would help if we did not need to specify it on the command line each time. Some integrations such as VSCode auto-download buildifier and auto-run it.

Fixes could include:

• if .buildifier-tables.json exists in root of workspace, use it
• add config option to .buildifier.json file with the path to tables.json (and whether it is an -add_tables or --tables)
• allow buildifier-tables.json content to be embedded inside .buildifier.json

Maybe I have missed something and this is already there -- if so it would be helpful to know. Thanks for a great tool!