Add support for config merges in Sheldon

When you're first setting up Sheldon, you'll likely have multiple
variants of the same file across several hosts. It's likely that each
variant has settings / snippets that offer value, and that you'd rather
not lose. This branch introduces support for a new Sheldon#merge,
allowing a user to merge their local version of a file with the "master"
version saved in Sheldon. This process can be repeated across all your
hosts until your "master" file contains the best bits of all your
different config variants.

lib/sheldon.rb

@@ -1,6 +1,7 @@
 require_relative "sheldon/brain"
 require_relative "sheldon/builder"
 require_relative "sheldon/helpers"
+require_relative "sheldon/file_merger"
 require_relative "sheldon/memory"
 require_relative "sheldon/sheldon"
 require_relative "sheldon/exceptions"

lib/sheldon/file_merger.rb

@@ -0,0 +1,25 @@
+class FileMerger
+  require 'git'
+  require 'byebug'
+
+  def initialize(brain, cue)
+    @brain = brain
+    @cue = cue
+    # @git_directory = File.join(brain.location, "git-temp")
+    @git_directory = File.join("/Users/dave/Desktop", "git-test")
+    @git_scratchfile = File.join(@git_directory, "scratch.txt")
+  end
+
+  def merge!(path_to_users_file)
+    setup_git_scratchpad
+  end
+
+  private
+
+  def setup_git_scratchpad
+    git = Git.init(@git_directory)
+    File.open(@git_scratchfile, "w") { |f| f.write("") }
+    git.add
+    git.commit("initial commit")
+  end
+end