Set variables in a metadata header

[oysandvik94]

Might be a useful idea to have a metadata header at the top of a file, seperated by "---", similar to markdown files, where someone can define variables such as hostname, token, etc.

These variables would then be set as environment variables in the context of the curl requests ran from that buffers, and can then also be referred to in curl commands as a regular environment variable.

Not sure how this solves hiding secrets though, as in #12 , but it seems useful nonetheless