Markdown writer: Add --bullet-list-marker argument option.

[nishidayuya]

Markdown reader understands all of bullet list marker (-, + and * character). But Markdown writer cannot use + and * character to bullet list marker.

This pull-request adds --bullet-list-marker argument to resolve it.

---

Input file is:

yuya@yoshiyuki|20:18:19|0% cat /tmp/a.textile
* foo
* bar
* baz

No --bullet-list-marker argument:
(equivalent to --bullet-list-marker=minus)

yuya@yoshiyuki|20:21:01|0% ./dist/build/pandoc/pandoc -t markdown /tmp/a.textile
-   foo
-   bar
-   baz

--bullet-list-marker=plus argument:

yuya@yoshiyuki|20:21:45|0% ./dist/build/pandoc/pandoc -t markdown --bullet-list-marker=plus /tmp/a.textile
+   foo
+   bar
+   baz

--bullet-list-marker=asterisk argument:

yuya@yoshiyuki|20:22:13|0% ./dist/build/pandoc/pandoc -t markdown --bullet-list-marker=asterisk /tmp/a.textile
*   foo
*   bar
*   baz