Add a flag to not escape HTML in specific cells

Add a `reactable.unsafe` function that will prevent the escaping of
any html string content using React's dangerouslySetInnerHTML method.

Refs #25
Add documentation for Reactable.unsafe

Refs #25
Merge pull request #26 from glittershark/gh/25/no_escape_flag

Add a flag to not escape HTML in specific cells
Fixes #25