Change MKDEXT_NO_INTRA_EMPHASIS to not start after alphanumerics

**No behavior should change if the NO_INTRA_EMPHASIS flag is not set.** With it set, the following behavior changes should be seen:  (The parentheses in the following examples can be replaced by any non-alphanumeric character.)

- "(\_italics\_)" is italic
- "this(\_is\_)weird" italicizes the "is"
- 'This is "\*\*bold\*\*"' emboldens "bold"

We were already checking for non-alphanumerics  on the closing boundary; that is, `"_this_()"` would have rendered as italicized even though the closing underscore wasn't followed by a space. This commit just brings the same behavior to the opening underscore.

Credit for this patch goes to @bjhomer: bdolman/sundown@b6434972a27ee677db433fbca6452fc78f7c0435 and is submitted here with his permission.