add: reinstall target to Makefile
Add an ignore file
fix: only implicit filenames must not be taken as is

This fix a bug when `Jg_template.from_file "/some/abs/path"`
was returning a file not found error for file "/some/abs/path/some/abs/path".

Not sure I understood the previous code, though.
fix: give original filename to Jg_interp.from_string for better err msgs
Simplify handling of optional arguments