Merge options with .gantree.cfg options

[#17575] [gantree] allow config through .gantree.cfg
Check if config file actually exists

[#17575] [gantree] allow config through .gantree.cfg
remove '.' from file name

[#17575] [gantree] allow config through .gantree.cfg
Merge pull request #9 from thebrowski/master-17575-gantree-allow-config

Master 17575 gantree allow config