Fix incorrect emphasis span calculation.

[snnz]

EmphasisInlineParser did not take into account the case of extra delimiter characters before the emphasis. For example:

**foo*

The supplied test produces the following result in the current version:

**foo*
```````````````````Result
paragraph    ( 0, 0)  0-5
literal      ( 0, 1)  1-1
emphasis     ( 0, 0)  0-5
literal      ( 0, 2)  2-4
```````````````````Expected
paragraph    ( 0, 0)  0-5
literal      ( 0, 0)  0-0
emphasis     ( 0, 1)  1-5
literal      ( 0, 2)  2-4

The emphasis intersects with the literal before it - obviously wrong. The effect of this can be seen in the Visual Studio editor: it displays the first asterisk as italics.

[xoofx]

Thanks!