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']
```

Start from the AutoFormatBuffer command and b:codefmt_formatter example in the issue, then inspect the existing formatter configuration and execution flow. Done means a buffer can configure multiple formatters and run them in the declared order, including the sortincludes then clang-format example.

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