Add nai theme.

themes/nai/README.md

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+## nai
+A minimalist fish theme which displays git branch and status information.
+
+
+#### Characteristics
+
+##### Left Prompt
+
+* Truncated CWD (just the current folder name)
+* Git branch and dirty state (if applicable)

themes/nai/fish_prompt.fish

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+# name: nai
+# Display the following bits on the left:
+# * Current directory name
+# * Git branch and dirty state (if inside a git repo)
+
+function _git_branch_name
+  echo (command git symbolic-ref HEAD ^/dev/null | sed -e 's|^refs/heads/||')
+end
+
+function _git_dirty
+  echo (command git status -s --ignore-submodules=dirty ^/dev/null)
+end
+
+function fish_prompt
+  set -l yellow (set_color yellow)
+  set -l green (set_color green)
+  set -l normal (set_color normal)
+
+  set -l cwd (basename (prompt_pwd))
+
+  echo -e ""
+
+  echo -n -s ' ' $cwd $normal
+
+  if [ (_git_branch_name) ]
+    set -l git_branch (_git_branch_name)
+
+    if [ (_git_dirty) ]
+      set git_info $yellow $git_branch
+    else
+      set git_info $green $git_branch
+    end
+    echo -n -s ' ' $git_info $normal
+  end
+
+  echo -n -s ' ' $normal
+
+end