Parse error on limit_req_zone with sync parameter

[mbibaud]

Describe the bug
Parsing an nginx configuration having limit_req_zone directives using the sync parameter causes crossplane to report an error: invalid number of arguments in "limit_req_zone" directive in

http://nginx.org/en/docs/http/ngx_http_limit_req_module.html#limit_req_zone

To Reproduce
Steps to reproduce the behavior:

1. Create an nginx config file with 'limit_req_zone $remote_addr zone=test_zone:10M rate=60r/s sync;'
2. Run crossplane parse on nginx config
3. See traceback

Expected behavior
Since sync is a valid parameter, I would not expect any errors returned.

Your environment

• Operating System Ubuntu 18.04
• Version of crossplane: 0.5.3)

Additional context
The following PR appears to fix the error and support both 3 and 4 parameters :
#77

[framer777]

#77 merged