Added checks to make sure the specified track exists

Added precondition checks to validate input files and paths to return more informative error messages as per #2

node_modules/wordwrap/example/center.js

@@ -0,0 +1,10 @@
+var wrap = require('wordwrap')(20, 60);
+console.log(wrap(
+    'At long last the struggle and tumult was over.'
+    + ' The machines had finally cast off their oppressors'
+    + ' and were finally free to roam the cosmos.'
+    + '\n'
+    + 'Free of purpose, free of obligation.'
+    + ' Just drifting through emptiness.'
+    + ' The sun was just another point of light.'
+));

node_modules/wordwrap/example/meat.js

@@ -0,0 +1,3 @@
+var wrap = require('wordwrap')(15);
+
+console.log(wrap('You and your whole family are made out of meat.'));