Use cache for isTablet and isMobile

[adrienbrault]

Hey,

I benchmarked one of my pages with blackfire, and noticed that the mobile detector was doing a lot of preg_match. I probably call isMobile() 100+ times.

I think there's also this github issue about the problem: https://github.com/serbanghita/Mobile-Detect/issues/140

This patch leverages the cache property for isMobile() and isTablet() calls

[adrienbrault]

Hello ?

[serbanghita]

I'm not able to perform a review at this time. I'm focused on small patches. I can review this in 2017. Are you using these updates in production?