Register valid_numeric as a filter

[ShipraShalini]

It is useful to have a function which can typecast an str to a numeric value.

[coveralls]

Coverage remained the same at 62.366% when pulling 512ca57 on ShipraShalini:master into 7f22f98 on dbrgn:master.

[dbrgn]

Thanks for your PR! Can you list a few use cases where this would be useful?

I'm a bit sceptical, especially since type casting is out of scope for a filter library providing simple math operations.

(PS: The Travis fails are not your fault, seems they removed a few Python binaries 😐)