Export a function instead of running w/shebang

To make integrating with plugin managers as easy as possible,
plugins/scripts shouldn't be sourced on their own; instead, they should
export functions.

README.md

@@ -31,9 +31,11 @@ git clone "https://github.com/xylous/gitstatus.zsh.git" gitstatus
 Add the following lines to your zshrc:
 
 ```zsh
+source path/to/installation/gitstatus.plugin.zsh
+
 function precmd()
 {
-    local gitstatus="$(path/to/installation/gitstatus.plugin.zsh)"
+    local gitstatus="$(gitstatus)"
     PS1="%F{blue}%~%F{default} ${gitstatus} $ "
 }
 ```

gitstatus.plugin.zsh

@@ -1,6 +1,4 @@
-#!/usr/bin/zsh
-
-function main()
+function gitstatus()
 {
     is_in_git_repository || return 1
 
@@ -149,5 +147,3 @@ function git_determine_color()
     typeset -g REPLY="$yellow"
     return 0
 }
-
-main