Apply **strong** / *emphasis* syntax for right after punctuations

[KSR-Yasuda]

For strong and emphasis syntax,
now it requires line beginning or some space before them.

**strong** aaa **strong**
*emphasis* aaa *emphasis*

Besides these, allow it is placed right after punctuations.

aaa (**strong**)
aaa (*emphasis*)

aaa,**strong**
aaa,*emphasis*

あああ （**最強調**）       # Wide (Japanese) paren
あああ （*強調*）

あああ、**最強調**
あああ、*強調*

あああ　**最強調**          # Wide (Japanese) space
あああ　*強調*

Especially, in Japanese, space is rarely used between words.
It is preferred to apply these syntax right after 、 or 。 (Japanese comma, period), even without any space.

[alerque]

Thanks for the contribution. I can totally see the logic for Japanese and even some of the Latin punctuation here, but I'm a bit worried this goes too far and will start catching false positives. In particular depending on the Markdown rendering in use it might be part of the requirement for these markups to have some space or other separator first. This avoids things like a clause,* more text where the asterisk might be a footnote marker or otherwise be literally rendered to not be mixed up with emphasis markup.

We need to look into how Pandoc, CM, GFM, and others handle this first and make sure we at least move closer to that handling and not farther away from it.

[KSR-Yasuda]

At least, Pandoc (v2.17.1.1) considers as I expected.

This avoids things like a clause,* more text

As Pandoc behavior, symbols should be considered as word edge, should not?
* occurence in non-symbol characters (e.g. foo*bar), it still treated as the character * itself.

If you want to use * character itself in such edge cases, I think you should escape it as \*.