Include assertion message as context (close #993)

Resolves #991

This expect statement

    expect("foo", "the foo").to.equal("bar");

now includes "the foo" as context for the failed match.

    1) reporter shows reason for failed assertion:

        the foo
        + expected - actual

        +"bar"
        -"foo"

lib/reporters/base.js

@@ -182,10 +182,13 @@ exports.list = function(failures){
     // actual / expected diff
     if ('string' == typeof actual && 'string' == typeof expected) {
       fmt = color('error title', '  %s) %s:\n%s') + color('error stack', '\n%s\n');
+      var match = message.match(/^([^:]+): expected/);
+      msg = match ? '\n      ' + color('error message', match[1]) : '';
+
       if (exports.inlineDiffs) {
-        msg = inlineDiff(err, escape);
+        msg += inlineDiff(err, escape);
       } else {
-        msg = unifiedDiff(err, escape);
+        msg += unifiedDiff(err, escape);
       }
     }
 
@@ -462,3 +465,5 @@ function sameType(a, b) {
   b = Object.prototype.toString.call(b);
   return a == b;
 }
+
+