ENH: Add strict parameter to filter

The strict parameter will ignore IDs that are not found in the distance
matrix.
TST: Add tests for strict filtering
DOC: Fix docstring
STY: Remove whitespace
DOC: Fix docstring details in filter

Thanks @gregcaporaso for catching these details.
STY: Remove whitespace
ENH: Change continue -> pass for clarity
Merge pull request #554 from ElDeveloper/dm-strict

Add strict argument to DissimilarityMatrix.filter