Support comma-first indentation

[robertoaloi]

Many Companies and libraries adopt a comma-first indentation in their code base. For example, an export list would look like:

-export([ completion/5
        , definition/3
        , did_open/4
        , did_save/1
        , did_close/1
        , document_symbol/1
        , exit/0
        , hover/3
        , initialize/2
        , references/3
        , document_highlight/3
        , shutdown/0
        , start_link/2
        , stop/0
        , workspace_symbol/1
        ]).

To increase adoption of a formatter, comma-first could be considered as a configuration option.