Implement YARA rules

To replace the current yaml signatures. This will allow us to create mroe powerful rules. For example to find GitHub API keys we would regex on `([a-f\d]{40})`, but currently that would produce a lot of false positives (it's a SHA1 hash). With a YARA rule we could do:

```
rule GitHubApikey
{
strings:
$re1 = /[a-f\d]{40}/
$re2 = /Authorization: token/
$re3 = /https://api.github.com/

condition:
$re1 and ($re2 or $re3)
}
```