Super merge.

Merge any number of exons or transcripts together. When merging transcripts,
merge overlapping and adjacent exons into one.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
When dragging several features (transcripts and / or exons), merge all into one.

If features are on opposite strand, ask which strand to create the resulting
feature on.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>