Support type attribute for <ol> element

[yhatt]

HTML5 can use type attribute for <ol> element to change the numbering type.

<ol type="I">
  <li>One</li>
  <li>Two</li>
  <li>Three</li>
  <li>Four</li>
  <li>Five</li>
</ol> 

And now, jsx-slack has supported type attribute to allow alphabetical and roman numbering.

Alphabetical numbering	Roman numbering
<Blocks> <Section> <ol type="a"> <li>One</li> <li>Two</li> <li>Three</li> <li>Four</li> <li>Five</li> </ol> </Section> </Blocks>	<Blocks> <Section> <ol type="I"> <li>One</li> <li>Two</li> <li>Three</li> <li>Four</li> <li>Five</li> </ol> </Section> </Blocks>

jsx-slack can render list items as mimicked text, but rendering new numbering types with keeping right-aligned is difficult with a traditional way. So we've updated how to generate spaces for indenting list item: Store the data of measured Slack font width by using Puppeteer, and generate the best indent string consisted of spaces from the width of numbering text.

By this change, the count of spaces for indent may be changed from <= v1.3.0, but would not expect visual regressions in the view of Slack client.

[yhatt]

Many browsers won't convert number to roman if the number is over than / equal to 4000.
https://jsfiddle.net/ukybLe5m/