tiler 0.2.0 release

• All three gdal2tiles* scripts have been updated to accept a command line argument when called by R that provides a path for any temporary files, i.e., tmp.*.vrt files created by the gdal2tiles* scripts. These were previously accumulating in the system temp folder. The new temporary directory is a sub-directory inside tempdir(). Therefore, it is cleaned up when exiting R. Nevertheless, tile also force deletes this subdirectory immediately after its internal system call to one of the gdal2tiles* scripts returns, so the temporary sub-directory does not even exist for the full duration of the tile call.
• Added functions tile_viewer and view_tiles and other supporting functions for generating HTML Leaflet tile preview web page.
• Added arguments to tile. tile now generates previewer by default.
• Added unit tests.
• Updated vignette.