Add basepairs to a sequence maintaining features in the right position

[isaacguerreir]

Apparently, if you add a basepair to a given sequence the features don't update start and end positions. I think will be great to have a function for this.

[TimothyStiles]

@isaacguerreir I was working on that and it was super hard to get appropriate updates for multi-location inserts. I'd be willing to accept something that handles circularity without the multi-location stuff.

https://github.com/TimothyStiles/poly/tree/sequence-methods