Added ability to insert variables defined in Gruntfile.js with [devcode:...

Adding ability to inject variables from `Gruntfile.js` with comments in the same style as before.

Example:

    <!-- devcode:=builddate -->
        Build date here
    <!-- endcode -->

with configuration

    options: {
        // ...
        vars: {
            builddate: '<%= grunt.template.today("isoDateTime") %>',
        }
    }

. The changes are backwards compatible. If the variable is not defined for the current configuration, the text between the comments are included instead.

This pull request solves issue #9.