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/ret/lib/positions.js

@@ -0,0 +1,17 @@
+var types = require('./types');
+
+exports.wordBoundary = function() {
+  return { type: types.POSITION, value: 'b' };
+};
+
+exports.nonWordBoundary = function() {
+  return { type: types.POSITION, value: 'B' };
+};
+
+exports.begin = function() {
+  return { type: types.POSITION, value: '^' };
+};
+
+exports.end = function() {
+  return { type: types.POSITION, value: '$' };
+};