Allow for adding grok patterns

The config file should provide a means of adding grok patterns, ether by naming a file in the usual format

```
PATTERN_NAME (my)?p[at]tern
```

or by just providing patterns inside the yaml file. The syntax could be

```
patterns: {
    PATTERN_NAME: '(my)?p[at]tern'
}
patternsource: /path/to/patternfile
```
