Custom `baseUrl` and `stylePath` parameters

[gavinr]

Describe the problem

From @corrigancd in #140:

Custom baseUrl and stylePath parameters - In our case, we require paths to the itemId that are not the ones hardcoded in the current implementation. So three parameters were added:

• baseUrl - The path to where the itemId resides (defaults to the previous portalUrl + <hardcoded-path-to-itemId> if not specified and useCustomUrlOrPath (see below) is not passed as true);
• stylePath - The path to the style json. This will be appended to after the itemId in the path when retrieving the vectorTileStyle file (defaults to the previous <hardcoded-path-to-style> value if not specified and useCustomUrlOrPath is not passed as true);
• useCustomUrlOrPath - Whether to use baseUrl and stylePath at all. If this is not passed, or passed as false, the previous functionality will happen, this includes the optional use of portalUrl.

Describe the proposed solution

Proposed solution from From @corrigancd in #140: here and here