Overlapping emphasis

[bert-github]

When two potential emphasis spans overlap, it is not always the first that takes precedence. (Violates rule 15.)

E.g.,

*foo _bar* baz_

is correctly parsed as

<em>foo _bar</em> baz_

but

_bim *foo _bar* baz_

is parsed as

_bim *foo <em>bar* baz</em>

(The first _, even though it is eventually not used for emphasis, still influences which delimiter is tried first.)