Add to_*** methods symmetrical to from_***

[nvaulin]

Hi,
Amazing package! I think it would be really convenient for many people to have not only read-functions (from_***), but write-functions as well (to_***). Because sometimes I have input data in 'indicators' format and i want it to be in 'contents', but for now diving deep into pandas is the only way to get it. Do you have any plans on making it or are there any ways already?

Thank you!