vanguard: normalized dates and numbers output.
vanguard: added support for scraping funds.

Now the `fund_id` argument supports comman separated values.
vanguard: only retrieve first table in result.

This avoids to extract the data from the previous result table as
the sites stores the previous query in session and displays the
previous result.
renamed FundItem to NavItem.
Added `vanguard_funds` spider to scrape all funds' information.