added theorem, lemma and corollary snippets (#474)

Author: rajitachandak

snippets/latex-mode/corollary

@@ -0,0 +1,8 @@
+# -*- mode: snippet -*-
+# name: corollary
+# key: corr
+# --
+\begin{cor}[${2:title}]
+  \label{cor:${1:label}}
+  $0
+\end{cor}

snippets/latex-mode/lemma

@@ -0,0 +1,8 @@
+# -*- mode: snippet -*-
+# name: lemma
+# key: lem
+# --
+\begin{lem}[${2:title}]
+  \label{lem:${1:label}}
+  $0
+\end{lem}

snippets/latex-mode/theorem

@@ -0,0 +1,8 @@
+# -*- mode: snippet -*-
+# name: theorem
+# key: thm
+# --
+\begin{thm}[${2:title}]
+  \label{thm:${1:label}}
+  $0
+\end{thm}