Eval grunt config template vars with --process

The utility of the `lineman config` command is limited when the user
needs to see what the actual interpolated value will look like, esp.
for file expansions. 

Example:

```
$ lineman config meta.banner
/*! <%= pkg.title || pkg.name %> - v<%= pkg.version %> - <%= grunt.template.today("yyyy-mm-dd") %> */
```

vs.

```
$ lineman config meta.banner --process
/*! An HTML/JS/CSS app - v0.0.1 - 2014-02-25 */
```
0.26.0