text: Title: Use XDG standard directories instead of hidden files in the home directory
  Body: Would it be possible to change (or, at as an option) the default locations of `~/.proplotrc` and `~/.proplot/fonts` and so on to the corresponding XDG standards?  A description may be found here: 
  
  https://wiki.archlinux.org/index.php/XDG_Base_Directory
  
  The standard has been gaining steam in the past five or so years; the idea is that apps don't have to litter the home directory with dot files, which makes backing up configuration files easier (not to mention, it keeps the home directory tidy!).
  
  The TLDR is that: 
  
  - The file `~/.proplotrc` should be moved to `~/.config/proplot/proplotrc`
  - The folders `~/.proplot/*` should be moved to `~/.config/proplot/*`
  
  (Actually, `~/.config` is more generally the value of the environment variable `XDG_CONFIG_HOME`.)
  
  Happy to submit a PR if there is interest. 
  