Add non-numeric post slug

[smerrill]

As mentioned by @davidneedham, changing slide order also changes slide IDs. We should generate non-numeric IDs from the filename so that CSS doesn't have to be changed as the order of the presentation changes.

We might just kill any [0-9_-]+ characters until we hit an alphanumeric character, such that 0000-1-1-intro.md gets a slug/ID of intro.