OCaml-stk : User's configuration

The user's configuration directory for Stk (named RCDIR below) is :

$(XDG_CONFIG_HOME)/stk if XDG_CONFIG_HMOE environment variable is defined,
or else $(HOME)/.config/stk.

The file RCDIR/config.json is a JSON file where the user can specify:

the theme to use :
```
{ ...
  theme = "dark",
  ...
}
```
the directories to look for fonts :
```
{ ...
  font_dirs: [ ... ],
  ...
}
```

Default configuration values are :

{
  "theme": "default",
  "font_dirs": [ "/usr/share/fonts", "/usr/local/share/fonts" ]
}

Defining themes

New themes for Stk interfaces can be defined in RCDIR/themes. Each theme is stored in a sub-directory, whose basename serves as theme name (which can then be used in RCDIR/config.json).

In a theme directory, each of the following files is optional :

preamble.css defines the preamble of the theme, i.e. usually variables and values used in the theme body (see Computing themes),
body.css defines the body of the theme,
options.json specify some options. By now, it can be used to indicate whether preamble and body must be prepended by preamble and body of the "default" theme, which makes creating new theme easier, not having to specify all required rules. If not present, the following content is used :
```
{ "prepend_default_preamble": true, "prepend_default_body": true }
```