add support for erl namespace

Summary:
In thrift we can specify
```
amespace erl somename
```
which overwrites the namespace prefix for the generated files.

Reviewed By: michalmuskala

Differential Revision: D68834666

fbshipit-source-id: 0ec8c001cd532a76b53f3d4ce7ba3d3009e88d77

fb303/thrift/fb303_core.thrift

@@ -26,6 +26,7 @@ namespace hack fb303
 namespace node_module fb303
 namespace go fb303.thrift.fb303_core
 namespace js fb303
+namespace erl something
 
 /**
  * Common status reporting mechanism across all services