Provide option to progressively show mask characters

[anthonator]

I'd like to be able to not display characters that are part of the pattern but the input has not reached yet.

For instance,

I'd like to be able to use this pattern:

(999) 999-9999

with the input:

555555

and get the output:

(555) 555

Currently I'd get the following output:

(555) 555-

If I were to pass in an empty string I'd get:

() -

[sibelius]

can you send a failing test for this?