[feature request] Support command to load given file

How about adding a `Given: filename`. That is, instead of writing the actual file contents in the vader file, one could import or load a desired file.

For me, this would make it easy to write e.g. a large `test.tex` file that I could use for many separate vader test files. An added convenience is that I can open the test file `test.tex` and manually do testing to find what is the real expected result.