Added a "format_extract" UDF.

The format_extract function takes three arguments:
 1. the FORMAT field from a VCF
 2. the sample field from a VCF
 3. the name of an attribute listed in the FORMAT field

All three are strings.

It returns a string which is the value of the attribute, from the sample field, whose name matches the third argument.
Adding the 'keyed_value' function.

The keyed_value UDF takes three arguments:
 1. a string with key=value pairs, separated by ';'s, i.e. the INFO field, and
 2. the name of a key
 3. a field which provides the default value.

Both are strings.  It returns a string, the value for the given key in the
provided pair list, or the default value if no such key exists.
Merge pull request #1 from tdanford/format-extract

Added a "format_extract" UDF.