Handle precision in format specifier.

- Added handling of precision to extract_format.
- Updated docs.
- Added test case for precision.
Updated extract_format docstring and added tests.

- Added [.precision] to extract_format docstring.
- Added test_precision + test_precision_fail tests.
    Note that the test_precision_fail tests that
    floats must have a leading 0. IS THIS CORRECT?
Merge pull request #23 from lkilcher/master

Handle precision in format specifier.