Add customOpener configuration option and handle clicks

[kkarpieszuk]

Fixes #3

Test steps:

• add select element which you want multiselect to be applied for
• add some <button id="foo">
• initialize multiselect with option customOpener: '#foo'
• click the button and make sure multiselect is open

You can see this in real use on this branch of WPForms:
https://github.com/awesomemotive/wpforms-plugin/pull/8341/commits/12d59d1b75ea3717ae43784bfd913f9df7282016

• go to WPForms > Entries > Select some form with entries
• on the entries list page click gear icon in right top corner of the table

[mahdiyazdani]

Thank you for your contribution here. Please see my comments below:

Based on the options property name, which suggests “customOpener”, I would recommend passing a single DOM element to the opener argument. This approach would eliminate the need to loop through multiple opener elements, as it is uncommon to have more than one opener for a single dropdown menu.

Additionally, please see the following non-blocker issues as well:

• It seems you didn’t build and compile the final package for distributed files. See the repository readme file in creating dist files.
• No demo is added to the demo website.
• No documentation is added to the “Settings” table on the demo website.

[kkarpieszuk]

Thank you @mahdiyazdani for the review. I applied all your suggested changes, including the update od the demo. Please review again

[mahdiyazdani]

Thank you @kkarpieszuk 🙌

[mahdiyazdani]

Thank you @kkarpieszuk 🙌