parser: allow root path specifications

parser: allow root path specifications

All a patch specification to be just '@' or '$' - this is mainly useful when
exporting the keys of the toplevel object to the shell.

Example:

    $ echo '{ "abc": 1, "def": 2 }' | jsonpath -e 'keys=@'
    export keys='abc'\ 'def';

Signed-off-by: Jo-Philipp Wich <jo@mein.io>