Use type_formatters to format datetime (if specified)

[jlstevens]

This PR gives a bit more flexibility when formatting dates by allowing a format string to be set as the formatter for datetime.datetime objects. For instance you can now do:

import datetime
hv.Dimension.type_formatters[datetime.datetime] = "%m/%y %Hh"

[coveralls]

Coverage decreased (-0.6%) to 54.693% when pulling a822871 on date_type_formatter into 81b0191 on master.

[jlstevens]

I don't intend to make any more changes to this PR. It can be merged whenever you are ready.

[philippjfr]

Looks fine to me. Merging.