Add "religious-icon" as a color variable

[jeisenbe]

Related to #3395

Changes proposed in this pull request:

• Add @religious-icon: #000000;
• Use this variable for place_of_worship, historic=wayside_cross and man_made=cross

In the future this could also be extended to other religious icons, such as wayside_shrine or other religious objects if they become defined in the database.

Test rendering is not shown, because the cartography is unchanged.
However, I have tested this commit and found that it works without problems.

[matthijsmelissen]

Thanks!