Support negative slices in `--cut`

It would be good to move to 0-based indexing: the slicing arithmetic
makes a bit more sense in the context of negative slices.
Clarify use of negative slices in `--cut`

See #33
More flexible `seqmagick backtrans-align`

Option to warn on failed translation; add file type inference to output;
change default alphabet to ambiguous DNA
Reindent
Indentation
Add "none" option to --fail-action
Accept "X" as translation for invalid codon

Catch TranslationError, KeyError
Merge branch '34-backtrans-align-illegal-codons'
Map .fq -> fastq
Update contrib
doc requirements
Merge branch 'master' of github.com:fhcrc/seqmagick