Add `append` and `prepend` commands

[loreanvictor]

These are useful for reusable recipes, as they might need to add to some files (for example, the README) without wholesale changing them. They should read some file, replace all variables in it, and append/prepend the content to some other file (without updating their variables).

prepend: header.md
to: README.md

Should it also support glob pattern? I am unsure, lets not do it for the first iteration at least.

[loreanvictor]

considering #4 , perhaps it is useful to have the input value be an expression instead of a file path:

prepend: "# Hallo!"
to: README.md

and have the combination of these two features handle appending / prepending files:

prepend:
  from file: header.md
to: README.md