draft-js-export-markdown Escaping all markdown characters

[SamiSaves]

This escapes all markdown related characters that are listed in markdown-escapes npm package.

It should solve issue #175

Before updating / adding tests, I thought it would be best to discuss this, because it probably needs some changes.

In my opinion the encoding should be enabled by default (With maybe an option to opt out). My reasoning is that for example, if you write character "#" without escaping it would be considered a header in markdown.