Load customized themes in addition to defaults

- New themes in `$BAT_CONFIG_DIR/themes` are now loaded *in addition* to
  the default themes (they may also override).
- The `Default.tmTheme` symlink is not necessary anymore.

This relates to #172

README.md

@@ -188,18 +188,12 @@ cd "$BAT_CONFIG_DIR/themes"
 # Download a theme in '.tmTheme' format, for example:
 git clone https://github.com/greggb/sublime-snazzy
 
-# Create a link to specify the new default theme
-ln -sf "sublime-snazzy/Sublime Snazzy.tmTheme" Default.tmTheme
-
 # Update the binary cache
 bat cache --init
 ```
 
 Finally, use `bat --list-themes` to check if the new themes are available.
 
-**Note:** Unlike for syntax definitions, adding custom themes currently *removes all default
-themes*. If you want to go back to the default themes, call `bat cache --clear`.
-
 ### Using a different pager
 
 `bat` uses the pager that is specified in the `PAGER` environment variable. If this variable is not