Add isSupported method

[itaisteinherz]

This fixes #337.

[bboydflo]

@zenorocha would be nice to have this merged

[xiwc]

Firefox38 can not use copy,but this check return true.

[itaisteinherz]

@xiwc According to MDN:

Before Firefox 41, clipboard capability needed to be enabled in the user.js preference file. See A brief guide to Mozilla preferences for more information. If the command wasn't supported or enabled, execCommand was raising an exception instead of returning false. In Firefox 41 and later, clipboard capability are enabled by default in any event handler that is able to pop-up a window (semi-trusted scripts).

Did you enable the clipboard capability in the user.js preference file? If you did, then I don't think that this is a problem with the method itself.

[xiwc]

@itaisteinherz Thank you for your answers. Is there a way to check clipboard capability is or not enabled as users usually do not know this switch feature.

[itaisteinherz]

@xiwc You could get the browser type and version using this module or this answer from Stack Overflow. Then, if the isSupported method returns true, the browser type is Firefox and the version number is lower than 41, the clipboard capability is enabled. If the browser type is Firefox, the version number is greater than 41 but the isSupported method returns false, the clipboard capability is disabled.

[xiwc]

@itaisteinherz got, thks.

[zenorocha]

Sorry for taking so long to merge @itaisteinherz. Thanks a lot!