additional config file paths

[TheSast]

Usually config files that are under a directory named after the related program are not mandatory to be dot prefixed themselves (often the name itself is omitted too).
It would be nice if erdtree checked these paths too:

$XDG_CONFIG_HOME/erdtree/erdtree.toml
$XDG_CONFIG_HOME/erdtree/config.toml
$HOME/.config/erdtree/erdtree.toml
$HOME/.config/erdtree/config.toml
%APPDATA%\erdtree\erdtree.toml
%APPDATA%\erdtree\config.toml

[solidiquis]

A new version is currently under development and I'll be sure to include these