Added snippets to help with math lectures

- snippets: Roman d, axes
- Vim: set fold to markers by default, jump to middle of line mapping

UltiSnips/texmath.snippets

@@ -1,9 +1,13 @@
-snippet al "Align (math)" b
+snippet ali "Align (math)" b
 \begin{align*}
-	$0 \\\\
+	$1 &= $2 \\\\
 \end{align*}
 endsnippet
 
+snippet al "Align line" b
+&= $2 \\\\
+endsnippet
+
 snippet pwf "Piecewise function" b
 \[
 	${1:name} = \left.
@@ -45,3 +49,15 @@ endsnippet
 snippet thus "Therefore" b
 $\therefore$ 
 endsnippet
+
+snippet rd "Roman d" i
+\mathrm{d}${1:x} $2
+endsnippet
+
+snippet xax "x-axis" i
+$x$-axis 
+endsnippet
+
+snippet yax "y-axis" i
+$y$-axis 
+endsnippet