support generic Display types inside of ANSIString

https://github.com/ogham/rust-ansi-term/pull/79
As mentioned in PR #11 

Supporting generic display types would be a useful feature in this crate. I'm looking into nu_ansi_term and if/how they implemented this. I noticed they merged a couple of the open PR's (either that or wrote some similar features themselves, I noticed it's 85 commits ahead)