Use original font if bold/italic version missing

MGMushParser's `attributedStringFromMush:font:color:` method currently crashes if it is provided a custom UIFont that does not have a bold or italic counterpart.

This fix allows bold and italic text to fall back to the default style should either font be missing.
Merge pull request #62 from nmostafavi/patch-1

MGMushParser should use original font if bold/italic version missing