Provide an easy way to have a file as Given

I am setting up Vader tests for Neomake, and would like to have an easy way to have an actual file with `Given`.

What about `Given test.sh` would write an actual file?

Otherwise I should probably use some wrapper functions for this.

Currently I am using this pattern:

``` vader
Given sh (Shell script with advanced shebang):
#! /usr/bin/env bash
true

Execute (Basic usage):
set noswapfile
write! test-file.sh
Neomake sh

Then:
AssertEqual neomake#GetEnabledMakers('sh'), ['sh', 'shellcheck']
```

Related: #71