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.