Add mechanism for chaining formatters

For some types of files you might want to run more than one formatter, for instance sort C++ includes (which clang-format doesn't handle) and then format with clang-format. Could codefmt support configuring a list of formatters instead of a single formatter? E.g.

``` vim
AutoFormatBuffer sortincludes,clang-format
let b:codefmt_formatter = ['sortincludes', 'clang-format']
```