Allow to customize input id

[eurek]

Hello,

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.

[coveralls]

Coverage increased (+0.2%) to 95.833% when pulling ee86ba2 on eurek:customize_hidden_input_id into 737dc2c on filestack:master.

[staturecrane]

@eurek This looks good, but please submit all PRs to be merged into the develop branch.

[eurek]

Done :)