add actions on process_lines

[zeegomo]

Draft of how Actions can be implemented. It uses maybe a little more memory than necessary because it duplicates the input line even if it's not modified but I believe that since the input is a single line and it goes out of scope as soon as it reads another line it's not very important to optimize this and this way it's easier to write/understand

[pietroalbini]

Left some comments on the main struct. Additionally, I'd love to see better expanded documentation for the struct and methods, and a buildtest to ensure the actions correctly work in a sandboxed build.