Add sequence search plugin

[cmdcolin]

Has been requested a couple times, can be complementary to genome wide search

Some user motivation

For sequence searching, in the past I have used IGVs "find motif" feature: https://software.broadinstitute.org/software/igv/motif_finder

Something like that would be very convenient for searching for primers and things like that without having to engineer it to use an external aligner (I would probably use bowtie for exact matches of small sequences). 

Even just a search of the viewing window, like JBrowse 1, would be very convenient.

from @malonge

[cmdcolin]

fixed by #3159