Allow inline styles to be rendered with * or _
Allow inline styles to be rendered with * or _

Some people prefer the __text__ over **text**
This pull request adds an option to implement these changes.
nickcernis/html-to-markdown,TomzxForks/html-to-markdown,thephpleague/html-to-markdown,thephpleague/html-to-markdown,jarnix/html-to-markdown,deadflowers/html-to-markdown,Joozt83/html-to-markdown,jeremykenedy/html-to-markdown,nickcernis/html-to-markdown