Merge pull request #648 from Learnosity/dprime

Add double prime symbol via `"` or \dprime
diff --git a/src/commands/math/basicSymbols.js b/src/commands/math/basicSymbols.js
index 8c80b32..7f44356 100644
--- a/src/commands/math/basicSymbols.js
+++ b/src/commands/math/basicSymbols.js
@@ -247,6 +247,7 @@
 LatexCmds[' '] = LatexCmds.space = bind(VanillaSymbol, '\\ ', ' ');
 
 LatexCmds["'"] = LatexCmds.prime = bind(VanillaSymbol, "'", '′');
+LatexCmds['″'] = LatexCmds.dprime = bind(VanillaSymbol, '″', '″');
 
 LatexCmds.backslash = bind(VanillaSymbol,'\\backslash ','\\');
 if (!CharCmds['\\']) CharCmds['\\'] = LatexCmds.backslash;