Add support for millisecond timestamps

[jdwyah]

It would be lovely if timestamps with milliseconds were accepted & parsed.
eg
df 1397478409577

also, this is awesome

[javi7]

yes please

[simongjh-zz]

+1 for milliseconds support

[geekbleek]

Yes please!

[geekbleek]

Added this - PR here: #10

[geekbleek]

Also added support for milliseconds in input as well: #11

[kerraway]

@geekbleek Thanks for your doing for this project. Could you release an updated version in your fork? Seemingly @mwaterfall didn't update this project.