Minor enhancements to rundiff script.

Minor enhancements to rundiff script.

util/rundiff:
    Make sure Algorithm::Diff is loaded only if you need it
    (so it's ok if you don't have it installed).
    Add command-line options for things you might want to change:
    - diff algorithm
    - # of context lines
    - # of lines of lookahead

--HG--
extra : convert_revision : b3ba02e1816eee286f47eb28ab578fa4734bac3a