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.