Merge pull request #396 from williamhogman/will-theme

Add 'will' theme. (double turnstile ⊨)

themes/will/README.md

+## Will
+Minimalist theme.
+
+![Screenshot](https://cloud.githubusercontent.com/assets/143746/6462675/60d171fe-c1ab-11e4-9434-8718a2139c79.png)
+
+#### 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