add 'will' theme

themes/will/README.md

+## Will
+Minimalist theme.
+
+#### Characteristics
+* The current working directory is displayed on the right-side
+* Hostnames are displayed if and only if we're on an SSH connection
+* Uses logical entailment as the prompt character

themes/will/fish_prompt.fish

+function fish_prompt
+  if test -n "$SSH_CONNECTION"
+    printf '%s ' $HOSTNAME
+  end
+
+  set_color red
+  printf "⊨"
+  set_color normal
+  printf " "
+end

themes/will/fish_right_prompt.fish

+function fish_right_prompt
+  set_color red
+  printf '%s ' (prompt_pwd)
+  set_color normal
+end