simple-markdown: make mailto: links comply with standard markdown

Summary:
[Standard markdown][1] preserves mailto: or MAILTO:
prefixes in the output of mailto autolinks, if the
author included the mailto: in the autolink

[1]: http://standardmarkdown.com/

Test Plan:
`make test`
ran the standard markdown tests and saw the following one pass:

input:

    <MAILTO:FOO@BAR.BAZ>

output:

    <p><a href="MAILTO:FOO@BAR.BAZ">MAILTO:FOO@BAR.BAZ</a></p>

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D12857
simple-markdown: Fix thead/tbody elements

Test Plan:
Tested in giant diff between simple-markdown and marked-react

Auditors: emily
semi-substate marked-react
simple-markdown: Fix paragraph end delimeter

Summary:
Paragraphs now allow spaces in the newline(s) before their
final newline. Apparently this is something people actually
write.

Test Plan:
`make test`

Auditors: emily
simple-markdown: output actual newlines not spaces

Summary:
For newlines, it now outputs '\n' instead of ' '.
This mostly makes diffing easier but also makes more
sense, given that newlines in text remain '\n's.

Test Plan:
 #yolo

Auditors: emily
simple-markdown: parse urls inside text as a url

Summary:
The first 'http' string inside text was being parsed as text
rather than the start of a url, causing urls to not be
recognized.

This change breaks the `text` rule on http so that this no
longer happens, and adds a test case (and fixes an old test
case).

 #fixingfuturebugs

Test Plan:
`make test`

Auditors: emily
I removed the implicit nature of adding and removing answers from the numeric-input widget. This commit also represents several
revisions based on Joel's comments.