Custom include

[jachym]

This patch is more complex and addresses more features:

1. Python3 conversion of the generate_style.py file, along with autopep script applied
2. Adds support for the INCLUDE directive of custom mapfiles on two places:
   1. In between of landuse and all linear and point layers: this way, you can create custom background maps with streets and labels on top of them
   2. On the top of all layers

Usage:

make STYLE=google BASE=my_mapfile.map OVERLAY=my_overlay.map

Example:

Aerial image put under all the streets and labels using BASE option

Sample base include file:

   TYPE RASTER
   CONNECTION "http://wmts1.geoportail.lu/opendata/service"
   CONNECTIONTYPE WMS
   METADATA
       "wms_srs"             "EPSG:3857"
       "wms_name"            "ortho_latest"
       "wms_server_version"  "1.3.0"
       "wms_format"          "image/jpeg"
   END

NOTE: this pull request is based on previeous #67

[jmckenna]

@jachym python3 was merged through #58, can you update your PR accordingly here?

[jachym]

will do

[jachym]

closing, since this is covered #72