Multi-stage formatting

[srhickma]

After #126, allow formatters to be pipelined to apply multiple stages of formatting.
For example, the formatting of a java file could have the following stages:
main formatting -> break long lines -> eliminate trailing whitespace -> done.

[srhickma]

It will be desirable to have all stages for a single file applied in succession, rather than running each stage on all files before proceeding. This will help to ensure that files are left in possibly invalid states for the smallest amount of time.

[srhickma]

Special care will need to given to ensure that tracking information is correct when using multiple formatters. We could use the sha256 of the combined specifications (in order, with some separators) to determine the combined version.