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

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

```