Add THEME_PROMPT_HOST for use in prompts

This switches the doubletime theme over to using a variable for
determining what to display in the prompt.  The original version's use
of a hard-coded `@local` means you would have that suffix displayed even
on remote machines.

themes/base.theme.bash

@@ -1,5 +1,6 @@
 #!/bin/bash
 
+THEME_PROMPT_HOST='\H'
 SCM_THEME_PROMPT_DIRTY=' ✗'
 SCM_THEME_PROMPT_CLEAN=' ✓'
 SCM_THEME_PROMPT_PREFIX=' |'