Allow colorization of history messages

History was always printed with `THEME_DEFAULT` we now use
`THEME_TEXT_HISTORY` which is accesible in theme files via
`main.text.history`.

Fix https://github.com/profanity-im/profanity/issues/1170

theme_template

@@ -22,6 +22,7 @@ statusbar.new=
 main.text=
 main.text.me=
 main.text.them=
+main.text.history=
 main.splash=
 main.time=
 input.text=