add real mobile capabilities to config

[t14]

This allows you to run test against real devices rather than emulators. https://www.browserstack.com/automate/physical-mobile-devices

[sinstein]

Hi @t14 , I have raised #315 for this very scenario. The real_mobile flag supported by BrowserStack is a boolean field (documentation on BrowserStack).

We also support 2 variants (real_mobile and realMobile) of the capability. This PR should no longer be necessary and can be closed.

[oradwell]

just found out you can achieve this by using extra_capabilities config like so:

browser_stack:
    capabilities:
        device: iPhone 7
        extra_capabilities:
            real_mobile: true