Add option to nastruct for specifying base pairing

[drroe]

Similar to the recently added guessbp option (#507) but give users a bit more flexibility. Something like:

nastruct basepair :995-1141 :1142-1288

The basepair arg could be specified multiple times. Could also make use of the bptype args for this.

NOTE: I think it's safe to assume all NA strands are laid out 5' to 3' but could think about adding some sort of prefix to indicate the ordering is flipped, something like:

nastruct basepair :995-1141 threeprime :1142-1288