add gp_enforce_rates cvar

- gp_enforce_rates defaults to 0
- If set to nonzero value, plugin will enforce the client cvars:
  rate 128000
  cl_cmdrate 128
  cl_updaterate 128
- If the following cvars are set to nonstandard values the plugin will
  display a warning message
  cl_interp_ratio 1.0
Merge pull request #83 from pmrowla/enforce-rates

add gp_enforce_rates cvar