YARA support

We now have (experimental) YARA support inside radare2.

If you are building from the latest git, you just have to install libyara, no need to recompile anything.

[0x00000000]> yara
Yara plugin
| add [path] : add yara rules
| clear      : clear all rules
| help       : show this help
| list       : list all rules
| scan       : scan the current file
[0x00000000]> 

Since you may not already have some rules, we bundled some defaults ones, for packers and crypto primitives.