Properly ignore files in chart folder

[joabakk]

When calling http://localhost:3000/signalk/v1/api/resources/charts I get
{".DS_Store":{"identifier":".DS_Store","tilemapUrl":"/charts/.DS_Store/{z}/{x}/{y}","type":"tilelayer","scale":"250000"},".gitkeep":{"identifier":".gitkeep","tilemapUrl":"/charts/.gitkeep/{z}/{x}/{y}","type":"tilelayer","scale":"250000"},"NW205_2":{"identifier":"NW205_2","name":"LYSEFJORDEN","description":"LYSEFJORDEN","tilemapUrl":"/mapcache/NW205_2","scale":100000}}

The two former are from OSX (very annoying .DS_Store files popping up) and .gitkeep (I assume from git repo). Files starting with .should be ignored as otherwise they end up in charts listings as undefined