Add simple coercions for numeric types

[RickMoynihan]

I think we need protocol based coercers for coercing strings into specific numeric types etc (and some general ones too).

Probably in a namespace like grafter.tabular.coercers or grafter.coercions.

• (->int "10") (->int 10)
• (->double "10.0") (->double 10.0)
• (->float "5.5")
• (->byte "128")
• ... one for all java/clj numeric types

And a more generic one that coerces more like you do there - by looking for .:

• (->number "10.21")

They should probably raise an exception on an invalid parse.