Add query parameters to start_requests' URL

[romifz]

ref #470

When a custom parameter is specified twice, Scrapy returns the last specified value only. So, in the example given in #470

-a qs=offset:123456789 -a qs=buyer:xyz

Scrapy would return qs=buyer:xyz. The implementation proposed here can take more than one parameter using , as separator:

-a qs=offset:123456789,buyer:xyz

[yolile]

@romifz I tested it with Paraguay and it works perfectly!
The only problem is when the value itself has a comma, eg scrapy crawl paraguay_dncp_releases -a qs=buyer.name:"Ministerio de Urbanismo, Vivienda y Habitat" the resulting URL has the parameter with the value just before the comma eg:
https://contrataciones.gov.py/datos/api/v3/doc/search/processes?tipo_fecha=fecha_release&fecha_desde=2010-01-01T00%3A00%3A00&fecha_hasta=2020-09-22T09%3A51%3A01&buyer.name=Ministerio+de+Urbanismo&page=4

and, could you add the new argument's documentation at BaseSpider please?

[romifz]

@yolile turns out that scrapy removes the quotes around the parameter values :(

I've modified the method to accept commas escaped using \, so for your example you can use:

scrapy crawl paraguay_dncp_releases -a qs=buyer.name:"Ministerio de Urbanismo\, Vivienda y Habitat"