Add fileContents template func to read a file into a template.

[jrasell]

Related #260

[jrasell]

@cconstantine this adds at least the fileContents function which allows you to call a file and its contents will be read into the template.

[cconstantine]

My reading of this code is that the file contents aren't treated like a template, just a big string. This is useful to me (Thanks!), but it would be significantly more useful to me if the file read by fileContents could be a template.

[jrasell]

@cconstantine reading the file contents as a template right off is tricky and not simple to solve. It would involve running the templater twice over the firstly the template read from file, then secondly over the job template so I want more time to think about this. This PR is only related to your issue, so wont close it.

As a potential work around for now, you could do something like the following:

levant render var='foo=bar'  -out=foobar.config foobar.config.tmpl && \
    levant deploy foobar.nomad 

Where the initial command renders out the config file, and the second then reads the rendered config file into the job file using the func added in this PR.