Rebuild index database does not respect selected source

[damb]

Hi,

cppman -h states that rebuilding the index database does respect the configured source i.e.

[...]
  -r, --rebuild-index   rebuild index database for the selected source, either
                        'cppreference.com' or 'cplusplus.com'.
[...]

In fact all sources are updated

cppman/cppman/main.py

Lines 135 to 138 in 2bb55e8

	sources = [('cplusplus.com', 'https://cplusplus.com/reference/', None),
	('cppreference.com', 'https://en.cppreference.com/w/cpp', '/w/cpp')]
	for table, url, path in sources:

I'm not sure which is the behaviour actually desired (Though, I assume it is the behaviour documented in the help.). Please let me know. I happily provide a PR.