Support multiple latex configurations. (#173)

* Support multiple latex configurations.

* Don't look for pdf2svg/dvisvgm unless they are actually required.

examples/doc_calligra.hs

@@ -0,0 +1,13 @@
+#!/usr/bin/env stack
+-- stack runghc --package reanimate
+{-# LANGUAGE OverloadedStrings #-}
+module Main (main) where
+
+import           Reanimate
+import           Reanimate.LaTeX
+import           Reanimate.Builtin.Documentation
+
+main :: IO ()
+main = reanimate $ docEnv $ staticFrame 1 $
+  center $ withStrokeWidth 0 $ withFillOpacity 1 $ scale 4 $
+  latexCfg calligra "calligra"