Option to make a plottranscript()

[seyoun209]

Hi Nicole,

Could you add an option in the plotTranscripts() function that allows users to highlight a transcript and select one or two specific transcripts to visualize? so then, I don't have to visualize other transcripts 👍
Would it be super helpful!!!!

Thanks a lot!!

[mikelove]

I would also like to use this. I've taken a shot, feel free to close the PR and take what you like, unless you think the code would break other things.