#1 Simplify --segment option and make <end> optional

[Eric Woudenberg]

This patch simplifies the --segment option.

Two changes:

1) The <prefix> field now specifies an entire file name, rather than something that has -0001.mp3 appended to it.
2) The <end> field is optional, and if omitted indicates that the segment runs to the end of the file.

So, e.g.: pymp3cut.py --segment out/myfile.mp3,26 myfile.mp3

Lops the first 26 seconds of myfile.mp3 and writes it to out/myfile.mp3

The new help sheet reads:

-s | --segment name,start[,end]

cuts a segment in the middle of the file begining at <start> and
ending at <end> ([[hh:]mm:]ss) and writes it to the file <name>. If
<name> does end in '.mp3', '.mp3' is added. Name can be - to send the
segment to stdout. If <end> is omitted the segment runs to the end of
the file.

[Eric Woudenberg]