Add timezone support to right prompt.

Allow overriding the right prompt datestamp timezone:

```
set -g theme_date_timezone America/Los_Angeles
```

README.md

@@ -94,6 +94,7 @@ set -g theme_title_display_path no
 set -g theme_title_display_user yes
 set -g theme_title_use_abbreviated_path no
 set -g theme_date_format "+%a %H:%M"
+set -g theme_date_timezone America/Los_Angeles
 set -g theme_avoid_ambiguous_glyphs yes
 set -g theme_powerline_fonts no
 set -g theme_nerd_fonts yes

fish_right_prompt.fish

@@ -1,5 +1,6 @@
 # You can override some default right prompt options in your config.fish:
 #     set -g theme_date_format "+%a %H:%M"
+#     set -g theme_date_timezone America/Los_Angeles
 
 function __bobthefish_cmd_duration -S -d 'Show command duration'
     [ "$theme_display_cmd_duration" = "no" ]
@@ -62,7 +63,7 @@ function __bobthefish_timestamp -S -d 'Show the current timestamp'
     or set -l theme_date_format "+%c"
 
     echo -n ' '
-    date $theme_date_format
+    env TZ="$theme_date_timezone" date $theme_date_format
 end
 
 function fish_right_prompt -d 'bobthefish is all about the right prompt'