fix issue created by #18 importing ggspatial (w/o adding ggspatial to Imports)

[elipousson]

I realized my last pull request introduced an error when running devtools::check() because it was importing ggspatial::layer_spatial but had kept ggspatial under Suggests. I added a check using rlang::check_installed() so the package can keep ggspatial under Suggests and avoid the need to have the package under Imports.

I also made several minor improvements to static_mapbox:

• feat: allow NULL zoom (or lon/lat) and NULL width/height for static_mapbox
• feat: set lon/lat based on centroid of location if location is not NULL and zoom is not NULL
• feat: validate zoom/bearing/pitch using match.arg (allowing NULL values)
• docs: minor updates to documentation

If you are OK with it, I'd also like to implement the attribution and logo parameters supported by the Mapbox static maps API. I can add that change to this pull request or a separate one after this is merged (since it may make sense to implement the same option to show or hide attribute to the addMapboxTiles function at the same time).

[walkerke]

looks good, thanks! yes, attribution would be great. I typically take care of that as a map caption which isn't always ideal so having that come through on the map itself would be helpful.