Use * instead of _ for italics

[Nic0S]

This fixes issues converting text with italics in the middle of the word, for example

aaaaaaitalicbbbb

Or when the space at the end of the word is italicized:

aaaaaitalic bbbb

Or when mixing bold / italic / bold+italic

aaaaitalicbolditalicitalicbbb

I have tested rendering the resulting markdown with both ReactMarkdown (https://github.com/remarkjs/react-markdown) and on iOS with the builtin AttributedString to confirm it renders correctly