Add fromFile interpolate helper

[LuBashQ]

Hi, as I have pointed in ovh/venom#822, it would be nice to have an interpolate helper that would read a file (either relative of absolute path) and return the contents of said file.

Doing so would open to cleaner assertions in venom, as the comparison values can then be stored in separate files and not added as variables. At least for the use cases I am facing in my day job, I prefer to store and have syntax highlighting and clearly stated changes/commits on said files, and those files could be shared as-is between several utilities.