Second and Third

Add support for \second and \third
diff --git a/src/commands/math/basicSymbols.js b/src/commands/math/basicSymbols.js
index 2173a29..756aa1c 100644
--- a/src/commands/math/basicSymbols.js
+++ b/src/commands/math/basicSymbols.js
@@ -239,6 +239,9 @@
 LatexCmds[' '] = LatexCmds.space = bind(VanillaSymbol, '\\ ', ' ');
 
 LatexCmds["'"] = LatexCmds.prime = bind(VanillaSymbol, "'", '′');
+LatexCmds["''"] = LatexCmds.second = bind(VanillaSymbol,'\\second ','″');
+LatexCmds["''"] = LatexCmds.third = bind(VanillaSymbol,'\\third ','‴');
+LatexCmds.backprime = bind(VanillaSymbol, "\\backprime ", '‵');
 
 LatexCmds.backslash = bind(VanillaSymbol,'\\backslash ','\\');
 if (!CharCmds['\\']) CharCmds['\\'] = LatexCmds.backslash;