Adds --valid-hangs to choose valid hanging indent

This option allows you to set specific indentation length(s) that override the default `[4]` or `[4, 8]`, for example `--valid-hangs 4` or `--valid-hangs 3,17` (if you're insane).

PEP8 deprecates the use of tabs, but states that "hanging indents *may* be indented to other than 4 spaces".

This might help with some tab use-cases, and also with the very common use of 8 spaces for hanging indent in standard statements (since this is recommended for `if: with: def:` blocks).

Means to replace #291 (which only added `--double-indent`, equivalent to `--valid-hangs 8`).