allow custom separator character for new unnested columns

[ryderdavid]

Hey there, this is a fantastic package! Would you be receptive to a contrib (or are you receptive to making the change) to support a custom separator character passed as a param?

something like... flat_table.normalize(df, sep='_') ?

[metinsenturk]

Yep. Sounds great! I will have a look at this.