Rationalize the parameter for choosing the theme

Currently it is possible to specify the output style theme with three different ways:

• -s --style for standard theme inside DATA_DIR/themes
• --base16 for standard theme inside DATA_DIR/themes/base16
• --config-file for an absolute .theme path file.

It would be easier to have only one parameter, for example:

--style=theme

theme can be an absolute path with .theme extension, otherwise it is considered as a theme name relative to DATA_DIR/themes. So:

• --style=/Users/username/mytheme.theme uses /Users/username/mytheme.theme
• --style=acid uses DATA_DIR/themes/acid.theme
• --style=base16/acid uses DATA_DIR/themes/base16/acid.theme

Note that even now the --plug-in parameter recognizes absolute path or relative from DATA_DIR/plugins folder.