Adding Files::register_for_mime_type() to associate a file analyzer

with a MIME type.

Whenever that MIME is detected, Bro will now automatically activate
the analyzer. The interface mimics how well-known ports are defined
for protocol analyzers.

This isn't actually used by any existing file analyzer (because we
don't have any yet that target a specific file format), but there's a
test making sure it works.

testing/btest/scripts/base/frameworks/file-analysis/bifs/register_mime_type.bro

@@ -0,0 +1,9 @@
+# @TEST-EXEC: bro -r $TRACES/http/get.trace %INPUT
+# @TEST-EXEC: btest-diff files.log
+
+event bro_init()
+	{
+	Files::register_for_mime_type(Files::ANALYZER_MD5, "text/plain");
+	};
+
+