* non builtins options require declaration using the decl command
* At the moment, only int and string options are supported, however
the goal of this change is to provide a consistent way to support
more complex options, namely lists and booleans
Now %sh{ ... } can access positional parameters through
standard shell construct, allowing simple handling of variable
parameters.
def command accepts -shell-params flag to use this facility.
when a kakoune releated env var used in a shell command, the ShellManager
tries to match it with given regex and the first that matches calls its
value retriever.
For this to be useful, EnvVarRetrievers now also take the variable
name in its parameters.
