Added option to skip git aliases definition

Fixes #947
  
An error occurred
 

modules/git/README.md

@@ -29,6 +29,10 @@ This setting affects all aliases and functions that call `git-status`.
 Aliases
 -------
 
+Aliases are enabled by default. You can disable them with:
+
+    zstyle ':prezto:module:git:alias' skip 'yes'
+
 ### Git
 
   - `g` is short for `git`.

modules/git/alias.zsh

@@ -26,6 +26,7 @@ zstyle -s ':prezto:module:git:status:ignore' submodules '_git_status_ignore_subm
 # Aliases
 #
 
+if ! zstyle -t ':prezto:module:git:alias' skip 'yes'; then
   # Git
   alias g='git'
 
@@ -180,3 +181,4 @@ alias gwc='git clean -n'
   alias gwC='git clean -f'
   alias gwx='git rm -r'
   alias gwX='git rm -rf'
+fi