Add author mappins for the git log

See https://git-scm.com/docs/git-shortlog#_mapping_authors

Makes for a prettier commit overview:
$ git shortlog -s -n --no-merges
  1673  Juan Linietsky
   696  Rémi Verschelde
   239  volzhs
   225  Andreas Haas
   209  Ignacio Etcheverry
   164  Daniel J. Ramirez
   164  Pedro J. Estébanez
   153  George Marques
   150  Paul Batty
   129  Poommetee Ketson
   122  Franklin Sobrinho
   120  marynate
   115  Bojidar Marinov
   107  Leon Krause
    96  Thomas Herzog
   ...

.mailmap

@@ -0,0 +1,26 @@
+Andrea Catania <info@andreacatania.com>
+Andreas Haas <liu.gam3@gmail.com>
+Andreas Haas <Hinsbart@users.noreply.github.com>
+Andreas Haas <entenflugstuhl@gmail.com>
+Ariel Manzur <ariel@godotengine.org>
+Ariel Manzur <ariel@okamstudio.com>
+Bastiaan Olij <mux213@gmail.com>
+Bernhard Liebl <poke1024@gmx.de>
+Bernhard Liebl <poke1024@gmx.org>
+Geequlim <geequlim@gmail.com>
+Ignacio Etcheverry <ignalfonsore@gmail.com>
+Indah Sylvia <ISylvox@yahoo.com>
+Jakub Grzesik <kubecz3k@gmail.com>
+Juan Linietsky <reduzio@gmail.com>
+Juan Linietsky <juan@okamstudio.com>
+Juan Linietsky <reduz@Juans-MBP.fibertel.com.ar>
+Julian Murgia <the.straton@gmail.com>
+Leon Krause <eska@eska.me>
+Leon Krause <eska@eska.me> <eska014@users.noreply.github.com>
+Marcelo Fernandez <marcelofg55@gmail.com>
+Mariano Javier Suligoy <marianognu.easyrpg@gmail.com>
+Paul Batty <p_batty@hotmail.co.uk>
+Poommetee Ketson <poommetee@protonmail.com>
+Saracen <SaracenOne@gmail.com>
+Thomas Herzog <therzog@mail.de>
+Zher Huei Lee <lee.zh.92@gmail.com>