added special variables

Adds variables like `${/filename}` that expand into special values, much like `__filename` in node.  The prefix character is designed to not conflict with normal variable names if/when they are supported.  I don't know where docs are supposed to go so here that are ...

### Special variables in snippets package

Add variables of the form `${Xname}` where the first character `X` is one of `/`, `:`, or `-`.  The suffix `name` must follow specific values or syntax.  Examples ...

### files
   - **${/filename}**   =>  /root/proj/lib/main.coffee
   - **${/dirname}**   =>  /root/proj/lib   
   - **${/basename}**   =>  main.coffee
   - **${/extname}**   =>  .coffee
   - **${/sep}**   =>  /
   - **${/deliminator}**   =>  :
   - **${/projectpath}**   => /root/proj
   - **${/project}**   => proj
   - **${/filenamerel}**   => lib/main.coffee
   - **${/dirnamerel}**   => lib
   - **${/line}**   => 27  (line number of cursor position)

### date & times
   - **${-any-moment-format}**  current time in any format provided by the [moment](http://momentjs.com/) npm package
      - ${-MMMM Do YYYY, h:mm:ss a} =>   November 22nd 2014, 2:24:26 am
      - ${-dddd}                    =>   Saturday
      - ${-MMM Do YY}               =>   Nov 22nd 14

### package.json
   - **${:path:in:project:json}**  Shows value from project.json file
      - ${:name}   =>  moment
      - ${:version} => 2.8.4
      - ${:repository:url}   =>  https://github.com/moment/moment.git