externally-defined criteria for keeping/dropping records

[nhoffman]

Let's add an option 'filter_outliers --filter-functions' that will allow the user to specify a file containing python code for some (optional) functions, each with the same signature: func(seq, info) where seq is a single sequence record, and info represents annotation (a dict) for the corresponding sequence. One or more of the following functions may be defined:

• keep_before(seq, info) - keep if True
• drop_before(seq, info) - drop if True

These override other criteria that are applied before outlier detection (so that we have more fine-grained control of what sequences are considered for outlier detection).

• keep_after(seq, info) - keep if True
• drop_after(seq, info) - drop if True

These override the results of outlier detection.

Comments? Questions?