Added a CustomScanner to wrap yr_scanner routines

[ncatlin]

This PR deals with the external variable support TODO by creating a scanner with compiled rules and then calling it using external variables. It reuses most of the memory scanning code from the usual scanner and follows the same API format as the other externs so the actual changes are relatively minor.

It can be used like this:
`
public static void ExternVariableTest()
{
string ruleString = "rule EXE_cloaked_as_TXT {\nmeta:\n\tdescription = "Executable with TXT extension"\n" +
"\tlicense = "https://creativecommons.org/licenses/by-nc/4.0/\"\n" +
"\tauthor = "Florian Roth"\ncondition:\nuint16(0) == 0x5a4d and filename matches /\.txt$/is}";

        byte[] fileContentsBuf = Encoding.ASCII.GetBytes("MZ_other_file_contents");

        using (YaraContext ctx = new YaraContext())
        {
            // Compile list of yara rules
            CompiledRules rules = null;
            using (var compiler = new Compiler())
            {
                //compilation will fail if the variable used by a rule isn't declared at compile time
                compiler.DeclareExternalStringVariable("filename");
                compiler.AddRuleString(ruleString);
                rules = compiler.Compile();
            }

            // Initialize the scanner
            var scanner = new CustomScanner(rules);

            //add string, long, double, bool external variables
            ExternalVariables externalVariables = new ExternalVariables();
            externalVariables.StringVariables.Add("filename", "ExecutableFile.txt");

            List<ScanResult> scanResults = scanner.ScanMemory(ref fileContentsBuf, externalVariables);
            Debug.Assert(scanResults.Count == 1);
    }

}
`