feat(git_stash): add configurable colors for stash segment

- Updated README to document `fg_color` and `bg_color` options for the `git_stash` segment.
- Modified segment implementation to use these options, falling back to theme defaults when not provided.

README.md

@@ -319,6 +319,11 @@ The options for the `time` segment are:
 
 - `format`: Format string as used by strftime function, e.g. `%H:%M`.
 
+The options for the `git_stash` segment are:
+
+- `fg_color`: Override the foreground color for the stash segment.
+- `bg_color`: Override the background color for the stash segment.
+
 ### Contributing new types of segments
 
 The `powerline_shell/segments` directory contains python scripts which are

powerline_shell/segments/git_stash.py

@@ -26,8 +26,8 @@ class Segment(ThreadedSegment):
         if not self.stash_count:
             return
 
-        bg = self.powerline.theme.GIT_STASH_BG
-        fg = self.powerline.theme.GIT_STASH_FG
+        bg = self.segment_def.get("bg_color", self.powerline.theme.GIT_STASH_BG)
+        fg = self.segment_def.get("fg_color", self.powerline.theme.GIT_STASH_FG)
 
         sc = self.stash_count if self.stash_count > 1 else ''
         stash_str = u' {}{} '.format(sc, RepoStats.symbols['stash'])