Removes dependency on fs-extra.

This should remove ~16 of the 25 dependencies of this project, which reduces the attack surface area and gets us down to only direct dependencies I believe.

See ethereum#361 (comment) for additional discussion.

I went ahead and removed the path check per thought from @chriseth, but I'm happy to put it back in if desired.

solcjs

@@ -3,7 +3,7 @@
 // hold on to any exception handlers that existed prior to this script running, we'll be adding them back at the end
 var originalUncaughtExceptionListeners = process.listeners("uncaughtException");
 
-var fs = require('fs-extra');
+var fs = require('fs');
 var os = require('os');
 var path = require('path');
 var solc = require('./index.js');
@@ -223,7 +223,7 @@ if (!output) {
   }
 }
 
-fs.ensureDirSync (destination);
+fs.mkdirSync(destination, { mode: 0o777, recursive: true });
 
 function writeFile (file, content) {
   file = path.join(destination, file);