Implemented AMBER_HEADER and AMBER_FOOTER variables

[Thesola10]

This fixes #344 and #672 by introducing an AMBER_HEADER variable that points to the customizable header file, as well as an AMBER_FOOTER variable to optionally append raw data to the end of the script.

Again, this is a proof-of-concept, and it stands to reason that it could be implemented in a better way, but this can serve as a starting point.

[b1ek]

i've edited this a bit to be more consistent with the rest of our codebase and i think this is ready to be reviewed & merged.

i also hardcoded newline between header and footer to prevent the first line being commented out if the user supplies their own header which doesn't have a newline at the end, like this:

long text with code

echo "hi!"

# my custom header
# amber version: 1.4.0echo hi!

[Mte90]

@Thesola10 can you update this pr with the latest changes?

[Ph0enixKM]

@Thesola10 are there any updates regarding this PR?

[Mte90]

@Ph0enixKM as I can remember he said that he doesn't have time anymore to work on the PRs.

The problem is that requires someone skilled with Rust othherwise I will already do it.