added clipping

[lskatz]

This is untested except that I know that there aren't syntax errors. Addresses #15

Adds --clip to remove any soft clipping and convert them to hard clipping

Examples (in theory):

• 174M76S gets converted to 174M76H and 76bp gets trimmed
• 174M10S10H gets converted to 174M20H and 10bp gets trimmed
• 10S174M gets converted to 10H174M, 10bp gets trimmed, and the position changes by 10bp

Reply in the conversation if you tested it rigorously or at all.