Fix bug in Bio::SearchIO::Exonerate with negative score

Exonerate can return negative scores, such as:
`vulgar: aly-miR169a 0 21 + contig15750__54-177 19 40 + -3 M 21 21`
(note the -3)

This was not corretly parsed by `Bio::SearchIO::Exonerate`, in the `next_result` method.

This patch tweaks the regular expression to account for a possible minus symbol '-' before the digits, and adds a unit test case.