Limiter improvements

Limiter now can be applied to any input/output, and can be set both in percentage or absolute value. 

```
# Replay 10% of traffic
gor --input-raw=":28021" --output-http="staging.com|10%"
```

Percentage based limiter should solve #113 as well. 

File input also have its own rate limiting algorithm and instead of dropping requests in can slowdown or speedup requests flow. This means that now you can actually do simple load testing with gor like this:

```
# Replay at 2x rate
gor --input-file="requests.gor|200%" --output-http="staging.com"
```

Load testing current works only for file input and only when you set limiter in percents. 

List of all changes:
* Refactored plugin registration
* All limiter related code moved from plugins. Now every plugin is wrapped to limiter while registering plugin.
* Added percentage based limiting
* File input have its own limiter allowing to it slowdown or speedup without dropping requests, available only when using percentage based limiter.
* Now you can run single tests in docker with make command: `make dtest ARGS=-test.run=LimiterTest1`