1. Fix not only *+- but also \ matched

Need not escape inside [] because backslash codes do not work inside [].
So pattern [\*+-] match also \.

2. Fix not only list but also horizontal are indented

If only space and three '*' or '-' character are in the line,
this line means horizontal item.
If current line is below horizontal item, it need not to indent.
Following example is horizontal item.

---
***
- - -
* * *

And list item must be specified space after [*-+].
Following example is list item.
* foo
- bar
+ baz

But following example is not list item.
*foo
-bar
+baz

Add indent test file for above bugs.

Fix #90

1. Added setlocal formatoptions-=c to disable automatically inserting bullets when auto-wrapping with textwidth.
2. Added indent/mkd.vim to support auto indent in list.