Make ordered lists preceded by paragraph parsed

Hello,

Previously, enabling the `:lax_spacing` option, if a paragraph was followed by an ordered list it was previously unparsed and was part of the paragraph but this is no more the case.

The fix is pretty straightforward, since the `parse_paragraph` method was checking through the `isalnum` function whether the first char on the next line was alpha-numeric but ordered list start with numerics, we just need to substitute the function with isalpha.

Fixes #311

Have a nice day.