Support non-default script path

[lucaswerkmeister]

Currently, the library assumes that the action API endpoint is found under /w/api.php, i.e. that the script path is /w:

wikibase-sdk/lib/wikibase-sdk.js

Lines 26 to 30 in d04dc7c

	instanceRoot = instance
	.replace(/\/$/, '')
	.replace('/w/api.php', '')
	instanceApiEndpoint = `${instanceRoot}/w/api.php`

This is not true for all wikis, and I think the library would be more universally useful if it supported other wiki setups as well. (For instance, I have several development wikis locally, with script paths /wiki1, /wiki2, etc.)

[lucaswerkmeister]

Also:

wikibase-sdk/lib/queries/get_entity_revision.js

Line 11 in d04dc7c

return `${instance}/w/index.php?title=Special:EntityData/${id}.json&revision=${revision}`

[maxlath]

solved by #78