feat(ss_framerate): Add config option for min/max frame rate.

config.js

     // The URL to the Firefox extension for desktop sharing.
     desktopSharingFirefoxExtensionURL: null,
 
+    // Optional desktop sharing frame rate options. Default value: min:5, max:5.
+    // desktopSharingFrameRate: {
+    //     min: 5,
+    //     max: 5
+    // },
+
     // Try to start calls with screen-sharing instead of camera video.
     // startScreenSharing: false,
 

react/features/base/tracks/functions.js

 
     const {
         constraints,
+        desktopSharingFrameRate,
         firefox_fake_device, // eslint-disable-line camelcase
         resolution
     } = store.getState()['features/base/config'];
                 constraints,
                 desktopSharingExtensionExternalInstallation:
                     options.desktopSharingExtensionExternalInstallation,
+                desktopSharingFrameRate,
                 desktopSharingSources: options.desktopSharingSources,
 
+
                 // Copy array to avoid mutations inside library.
                 devices: options.devices.slice(0),
                 firefox_fake_device, // eslint-disable-line camelcase