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)
}