Improve parsing of browser desired capabilities

Improve parsing of browser desired capabilities:

• Can parse both .py and .json files
• Can handle nested options

More info: SeleniumBase/examples/capabilities/ReadMe.md

Sample cap file 1: (Generated from https://www.browserstack.com/automate/capabilities)

desired_cap = {
    'os': 'OS X',
    'os_version': 'High Sierra',
    'browser': 'Chrome',
    'browser_version': '77.0',
    'browserstack.local': 'false',
    'browserstack.selenium_version': '3.141.59'
}

Sample cap file 2: (Generated from https://wiki.saucelabs.com/display/DOCS/Platform+Configurator#/)

capabilities = {
    'browserName': 'firefox',
    'browserVersion': '70.0',
    'platformName': 'macOS 10.13',
    'sauce:options': {
    }
}