Add support for /startups?filter=raising endpoint

Although the enpoint is documented as an endpoint for fundraising
companies, it looks more like an index for the startups which supports
a filter. Because of that I opted for calling the method #all_startups,
instead of #get_fundraising or similar, and accept a :filter param
which defaults to 'raising'. This way, if in the future this enpoint
accepts more filters, we'll be covered.