Allow theming to be disabled (eg for old terminals)

* Only load a theme file if `bobby` has been set
 * Only alter `PS1` if `\[\]\[$(scm_char)\]\[\]\[$(scm_prompt_info)\]\[\]\[$(rvm_version_prompt)\] \[\]\h \[\]in \[\]\w \[\]\[\n\[\]→\[\] ` has a value

lib/appearance.bash

@@ -8,4 +8,6 @@ export GREP_COLOR='1;33'
 export LSCOLORS='Gxfxcxdxdxegedabagacad'
 
 # Load the theme
-source "$BASH/themes/$BASH_THEME/$BASH_THEME.theme.bash"
+if [[ $BASH_THEME ]]; then
+    source "$BASH/themes/$BASH_THEME/$BASH_THEME.theme.bash"
+fi