Be more conservative of casting text that contains dates to date and datetimes

[fgregg]

Currently, if the a field contains text that can be cast to a date or datetime by parsedatetime, agate will cast the entire field to a date or datetime, even if there are other parts of the field.

For example:

> printf "foo\n720(38-4)31A-1.1(A)" | csvlook
|                 foo |
| ------------------- |
| 2040-01-01 07:20:00 |

and

> printf "COMMENT\nMC 5.7.10 Per Dorothy Carroll\ntesting workgroup fix - 4/7/2010 - Marcy Liberty" | csvlook
|    COMMENT |
| ---------- |
| 2010-05-07 |
| 2010-04-07 |

This PR addresses this inappropriate casting and adds tests.

Releated to wireservice/csvkit#910 and wireservice/csvkit#911

[fgregg]

Thanks @jpmckinney, what's the review and approval process here?

[nbedi]

Looks good! Thanks for opening this. Merging now, will be in the next release.