Add a Git .mailmap with my new name

mgol · mgol · commit 6ac8ebafba47 · 2017-10-04T12:55:41.000+02:00
In this way my past contributions are mapped correctly.
diff --git a/.mailmap b/.mailmap
@@ -0,0 +1 @@
+Michał Gołębiowski-Owczarek <m.goleb@gmail.com>

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	`+Michał Gołębiowski-Owczarek <[email protected]>`