Do not include whitespace when applying inline markdown styles

[Rosey]

If you test out markdown on a gist, markdown like this:

**Test **

(with trailing or leading whitespace) is not properly parsed. (examples here: https://gist.github.com/Rosey/3f848d77b46f7ab337a53211117acd87)

This change only surrounds the text with markdown inline styles and preserves the whitespace, but doesn't attempt to style it, fixing the issue of incompatibility with markdown parsers.

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I'm not super confident about the efficiency of my regular expression here. It works, but I am worried that it is not optimal. Perhaps someone with better regex-fu will know if it can be improved performance-wise.

[binarybaba]

@Rosey are you planning to fix the conflicts? Your PR most probably also solved https://github.com/sstur/react-rte/issues/127