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.
diff --git a/themes/base.theme.bash b/themes/base.theme.bash
index e77d1d2..56629b9 100644
--- a/themes/base.theme.bash
+++ b/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=' |'
diff --git a/themes/doubletime/doubletime.theme.bash b/themes/doubletime/doubletime.theme.bash
index e1f91f5..55df693 100644
--- a/themes/doubletime/doubletime.theme.bash
+++ b/themes/doubletime/doubletime.theme.bash
@@ -25,7 +25,7 @@
   history -c
   history -r
   PS1="
-\t $(scm_char) [\[$blue\]\u@local\[$reset_color\]] \w
+\t $(scm_char) [\[$blue\]\u@${THEME_PROMPT_HOST}\[$reset_color\]] \w
 $(doubletime_scm_prompt)\[$reset_color\] $ "
   PS2='> '
   PS4='+ '