Treat empty as zero

This pull request adds a new option to the library: `treatEmptyAsZero`.

Currently in this library, when focusing on an empty field, the masking library assumes the user intends "blank" to mean 0. I needed empty to mean "nothing" and allow the user to specify either "nothing" or 0 as the submitted value.

Here, setting `treatEmptyAsZero` to `false` allows you to delete a `0.00` masked value and allows the user to enter either a blank input or zero.