Add 'ignoreSelect'

[francisdaigle]

Add option to disable automatic closing of dropdown after selection made by exposing paper-menu-button ignoreSelect.

Fixes #53, #171

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[francisdaigle]

I signed it!

[googlebot]

CLAs look good, thanks!

[jpradelle]

I was about to do that PR the same way, this is really needed, can someone merge it ?
There are several opened PR for the same need:
#177
#186
#230
#255

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CLAs look good, thanks!