Allow for "external files" in blacklist/whitelist
Allow for "external files" in blacklist/whitelist

This will do two things:
- Add a new option to include entries  from files using a special directive `- "!file /path/to/list"`, for example:
- use a `set` instead of a `list` in the blacklist/whitelist entries, which is much faster with larger sets and deduplicates automatically.
```
blacklist:
- "evil.com"
- "hacker.org"
- "virus.cc"
- "!file /tmp/black.txt"
- "!file /tmp/black2.txt"
```
You can mix between the list entry definition formats, or use either one.
Feedback is welcome :)
Yelp/elastalert,rprabhat/elastalert