Fix parse accepting strings with trailing characters

[djhoese]

Pointed out by @sfinkens on slack. Parse should not allow trailing characters.

This fixes this by adding ^ and $ to the regular expression for extract_values and allowing it to be turned off with full_match=False to the parse function.

[coveralls]

Coverage increased (+0.1%) to 94.393% when pulling f7f2aa2 on djhoese:bugfix-full-match into d6c0b81 on pytroll:master.

[mraspaud]

LGTM. Thanks for making the full_match optional

[pnuu]

LGTM