Add onInput callback

[ankurk91]

Hi,

If you think #656 is too complicated and might be breaking for users then this PR might be helpful.

The PR will add another callback onInput which be called without checking for oldValue.
This will really help developers like me to port this awesome library to other js frameworks for example -
https://github.com/ankurk91/vue-jquery-mask

[igorescobar]

@ankurk91 thanks for your contribuition!

what's the difference between this and the native input event?

[ankurk91]

The onInput callback will be called after this library has finished its internal calculations and then external javascript can perform actions like updating the v-model.

The native input event listener might be called before the library finished its internal processing.
The library is also listening to same input event

The onInput callback ensures that there is no pending task from the library and external javascript can proceed with value returned by callback.