Rewrite build config parsing to me shorter, cleaner and use some standard config format

[igor-petruk]

YAML or TOML are good candidates

[igor-petruk]

Something like this if TOML

script_src="script.ml"
build_cmd="dune build script.exe"
target_bin="@@@/_build/default/script.exe"

[[file]]
name="dune"
contents="""
(executable
     (name script)
      (libraries lwt.unix))
"""

Inside of the block

#  scriptisto-begin
#  <-- here
#  scriptisto-end

[igor-petruk]

Might look ugly for Rust, if we have TOML inside of TOML

[igor-petruk]

Oops, forgot to reference 7f1286b