ggcall v0.3.0.9000

Added support for ggplot related patchwork operators.
Updated ggcall attribute of ggcall() so it is a language object not a list of such.
Improved ggcall_add_assignments input validation for a call argument, it has to be a symbol.
Edited README file.

ggcall v0.2.0

Added plot_call_with_assignments function, ggcall can be extended with connected variables assignments.
Edited README file with more clear examples.
Added pre-commit.
Added coverage.

ggcall v0.1.0

First release.