rmapshaper 0.5.0

This is a fairly major release with much of the internal plumbing changed. I have tried to keep user-facing changes to a minimum, but please report any issues to https://github.com/ateucher/rmapshaper/issues.

• Switched to using the geojsonsf package instead of geojsonio for object conversion (#118).
• Updated the bundled mapshaper version to v0.6.25 (#130).
• Dropped support for geojson_list objects. This was a rarely-used class from the geojsonio package (#118).
• Arguments force_FC, sys, and sys_mem are now passed to apply_mapshaper_commands via ... rather than explicitly, so they are now documented in the ... section of each function. This may break some existing code if you were passing values to these arguments by position rather than by name, especially using force_FC in ms_simplify as it was not at the end of the argument list. It may also change the class of the return value for some input classes and functions (such as ms_lines and ms_innerlines) as force_FC will inherit the default TRUE for all functions.
• Added quiet argument to silence mapshaper console messages when using sys = TRUE. This can be controlled globally with options("mapshaper.sys_quiet" = TRUE) (#125).
• Added ability to globally set the system memory when using the system mapshaper via options("mapshaper.sys_mem"=X), where X is the amount of memory in GB.