Add Discord Rich Presence Support (#3883)

* Initial Discord RPC support

Build with Discord Presence ON

Fix RPC detection

Fix Time elapsed on pause; will now continue to count.

* Fix CI builds with compile flag

Addressed reviews

Fix silly mistakes

Fix 'Not in-game' display

class instead of namespace

Fix

Revamped

remove redundant code

Using Pimpl pattern

* Implement Null class

* Fix config updation

* Addressed All Reviews

* externals/discord-rpc : Updated to latest commit

.travis/common/travis-ci.env

@@ -13,3 +13,4 @@ TRAVIS_TAG
 
 # citra specific flags
 ENABLE_COMPATIBILITY_REPORTING
+USE_DISCORD_PRESENCE