Add an option to generate GError codes

Add the --error option to generate a GError code, with the associated
domain.

For example, to generate a class FooBar with error code
FOO_BAR_ERROR_QUUX_FAILED, use:

  $ gobjectogen --namespace Foo --error quux-failed FooBar

This option can be invoked multiple times.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Fix some warnings reported by flake8

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Add options to set legal information.

Add three command line options to set legal information:

* --author: set the author of the program
* --description: set the description of the program
* --license-file: set path to file containing license notice

The information is used to generate the legal disclaimer for the source
files.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Fix escaping of special characters

By default, Pystache is configured for generating HTML, so it escapes
some special characters, like '<'. The rendering code has been updated
to prevent escaping of such characters, as C code is generated, not
HTML.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Add option to generate boxed type.

GBoxed is a generic wrapper mechanism for arbitrary C structures.

The program now features a new command line option (--boxed) to generate
the source code for such a type.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>