Add new pyenv segment to show active python version

This commit provides a new segment `pyenv` to show current python environment.
This prompt can be used in place of `virtualenv` segment. However, it is
targeted at users of [pyenv](https://github.com/yyuu/pyenv) and not meant to
replace the `virtualenv` segment.

The prompt works by parsing output of `pyenv version` and displaying the first
word of the output as segment text. The design (color etc.) is same as the
`virtualenv` segment including the icon. The segment would need to be
revisited if `pyenv version` changes its output format.

Tested on my personal machines (Mac OSX 10.11.4 and Ubuntu 15.04).

README.md

@@ -116,6 +116,7 @@ The segments that are currently available are:
 * **Python Segments:**
     * `virtualenv` - Your Python [VirtualEnv](https://virtualenv.pypa.io/en/latest/).
     * [`anaconda`](#anaconda) - Your active [Anaconda](https://www.continuum.io/why-anaconda) environment.
+    * `pyenv` - Your active python version as reported by the first word of [`pyenv version`](https://github.com/yyuu/pyenv). Note that the segment is not displayed if that word is _system_ i.e. the segment is inactive if you are using system python.
 * **Ruby Segments:**
     * [`chruby`](#chruby) - Ruby environment information using `chruby` (if one is active).
     * [`rbenv`](#rbenv) - Ruby environment information using `rbenv` (if one is active).

powerlevel9k.zsh-theme

@@ -935,6 +935,22 @@ prompt_virtualenv() {
   fi
 }
 
+# pyenv: current active python version (with restrictions)
+# More information on pyenv (Python version manager like rbenv and rvm):
+# https://github.com/yyuu/pyenv
+# the prompt parses output of pyenv version and only displays the first word
+prompt_pyenv() {
+  local pyenv_version="$(pyenv version 2>/dev/null)"
+  pyenv_version="${pyenv_version%% *}"
+  # XXX: The following should return the same as above.
+  # This reads better for devs familiar with sed/awk/grep/cut utilities
+  # Using shell expansion/substitution may hamper future maintainability
+  #local pyenv_version="$(pyenv version 2>/dev/null | head -n1 | cut -d' ' -f1)"
+  if [[ -n "$pyenv_version" && "$pyenv_version" != "system" ]]; then
+    "$1_prompt_segment" "$0" "$2" "blue" "$DEFAULT_COLOR" "$pyenv_version" 'PYTHON_ICON'
+  fi
+}
+
 ################################################################
 # Prompt processing and drawing
 ################################################################