Added support for eager autocompletion

[jyardin]

Added a parameter autoCompletion in the MaskTextInputFormatter with two values:

• MaskAutoCompletion.lazy: the current behaviour, where unfiltered characters are added once the following filtered character is input.
  Ex: with the mask "#/#", "1" then "2" output "1" then "1/2"
• MaskAutoCompletion.eager: the new behaviour, where unfiltered characters are added when the previous filtered character is input.
  Ex: with the mask "#/#", "1" then "2" output "1/" then "1/2"

This fixes #44

[politebarista]

Thank you so much for your tweaking! Maybe you should erase the last character entered by the user, instead of jumping to the last character when you press backspace

[samuele-besoli-mndl]

When will we have this new update?

[scopendo]

Thank you for this @jyardin – I'm updating my pubspec.yaml to refer to your branch whilst this PR remains open.

@siqwin – I wonder if it is worth seeing if @fluttercommunity would like to take over the maintenance of this project. Your input formatter-based approach to masked input is superior in my opinion to other packages that introduce a custom text editing controller or alternative to TextField/TextFormField.

[siqwin]

Thanks