Allow to customize input id

[eurek]

Hi,

Sorry for the disturb of the last pull request #177.

I remind you what the problem:

We have multiple filestack_field in the same page with the same input-id.
get_filestack_field_button returns always the same id for the input, and this is a problem.
Here we can customize every id of your input.

Regards

[coveralls]

Coverage increased (+0.2%) to 95.833% when pulling 25072b8 on eurek:customize_hidden_input_id into f59d6e5 on filestack:develop.

[staturecrane]

Looks okay