Made insertion motif optional, expanded valid insertion sequence check and added support for target site deletions

[ramonamerong]

I had simulated mobile element insertions using BAMsurgeon for my project but encountered the following problems:

• The insertion motif did not work as intended so I made it optional.
• Any other nucleotide notation other than ACGT was not supported, so I added ambiguous nucleotides.
• While target site duplications are supported, target site deletions are not as (they occur more rarely). Therefore I made target site deletions possible if you set the TSD length to a negative value (e.g. -9).

I made this push request as I think others could benefit from these changes.