Added lambda theme which includes rbenv helpers

Characteristics

- Working directory
- Git branch and dirty state (if inside a git repo)
- Rbenv Ruby Version
- Rbenv Ruby Gemset

themes/lambda/README.md

+## Lambda
+
+A theme inspired by [oh-my-zsh](https://github.com/robbyrussell/oh-my-zsh/blob/master/themes/lambda.zsh-theme)'s [lambda](http://zshthem.es/screenshots/lambda.png) theme.
+
+![](http://f.cl.ly/items/0s2B1n3F0I31340E273G/Screen%20Shot%202015-02-05%20at%2010.50.06%20am.png)
+
+#### Characteristics
+
+* Working directory
+* Git branch and dirty state (if inside a git repo)
+* Rbenv Ruby Version
+* Rbenv Ruby Gemset

themes/lambda/fish_prompt.fish

+# name: lambda
+# ---------------
+# Based on oh-my-zsh lambda theme
+#
+# Display the following bits on the left:
+# - Working directory
+# - Git branch and dirty state (if inside a git repo)
+#
+# Display the following bits on the right:
+# - Rbenv Ruby Version
+# - Rbenv Ruby Gemset# - Current directory name
+
+function _git_branch_name
+  echo (command git symbolic-ref HEAD ^/dev/null | sed -e 's|^refs/heads/||')
+end
+
+function _is_git_dirty
+  echo (command git status -s --ignore-submodules=dirty ^/dev/null)
+end
+
+function fish_prompt
+  set -l blue (set_color blue)
+  set -l green (set_color green)
+  set -l normal (set_color normal)
+
+  set -l arrow "λ"
+  set -l cwd $blue(basename (prompt_pwd))
+
+  if [ (_git_branch_name) ]
+    set git_info $green(_git_branch_name)
+    set git_info ":$git_info"
+
+    if [ (_is_git_dirty) ]
+      set -l dirty "*"
+      set git_info "$git_info$dirty"
+    end
+  end
+
+  echo -n -s $cwd $git_info $normal ' ' $arrow ' '
+end

themes/lambda/fish_right_prompt.fish

+function _ruby_version
+  echo (command rbenv version-name | sed 's/\n//')
+end
+
+function _ruby_gemset
+  echo (command rbenv gemset active ^/dev/null | sed -e 's| global||')
+end
+
+function fish_right_prompt
+  set -l red (set_color red)
+  set -l normal (set_color normal)
+
+  set ruby_info $red(_ruby_version)
+
+  if [ (_ruby_gemset) ]
+    set -l ruby_gemset $red(_ruby_gemset)
+    set ruby_info "$ruby_info@$ruby_gemset"
+  end
+
+  echo -n -s $ruby_info $normal
+end