improvement: add parser for format strings (#1021)

This PR implements the parser of format strings described in #624.

src/config.rs

@@ -305,7 +305,7 @@ impl Default for SegmentConfig<'static> {
  - 'italic'
  - '<color>'        (see the parse_color_string doc for valid color strings)
 */
-fn parse_style_string(style_string: &str) -> Option<ansi_term::Style> {
+pub fn parse_style_string(style_string: &str) -> Option<ansi_term::Style> {
     style_string
         .split_whitespace()
         .fold(Some(ansi_term::Style::new()), |maybe_style, token| {