Use option[:id] for label's *for* attribute

[pinkahd]

Don't know if you still accept fixed for legacy-2.7 branch.
But I've ported the fix #357 for support id's on labels for the
legacy version of the gem.

Original PR message

If provided, use option `id` to specify `for` attribute on label
for (text, email...) fields, checkbox, radio and select.
Solves #342 issue and supercede #221 and #343 pull requests.

Partially solves #251 (only the part regarding the id).

[bootstrap-ruby-bot]

	1 Warning
⚠️	Please update CHANGELOG.md with a description of your changes. If this PR is not a user-facing change (e.g. just refactoring), you can disregard this.

Here's an example of a CHANGELOG.md entry (place it immediately under the * Your contribution here! line):

* [#507](https://github.com/bootstrap-ruby/bootstrap_form/pull/507): Use option[:id] for label's *for* attribute - [@pinkahd](https://github.com/pinkahd).

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[lcreid]

Sorry for the delay in responding. I'd be happy to merge this PR. I'm wondering if you know why we get the error about mocha methods in the Travis tests for Rails 4.2+? I'd feel better if we fix the test failures, because I suspect a lot of people still depend on this branch to work. I don't expect that your change will have broken anything, but it's happened to me lots of times.

If you don't feel comfortable trying to fix the test failures, just let me know and I'll try to make time to see what I can do.

[lcreid]

Not knowing how else to do it, I created a new PR #525 to fix the build problems.

@pinkahd Thanks very much for the original PR!