Add ability to create blog posts with custom JS and styles.

[itsjohncs]

We can potentially have the Phial app build a directory for each post that includes a collection of assets (and one HTML file). Then Gulp can just go through and combine everything.

This will let people add frontmatter to their posts pointing at the various assets they want and Gulp can handle the rest.

[itsjohncs]

@WChargin requested the ability to add in the KaTeX library to his post, which I can support through this.

[xymostech]

:+1:

[wchargin]

KaTeX should be satisfied by #33 and #34 together.

[itsjohncs]

For posterity, some of the requirements of this issue have been satisfied by the work above. But adding assets that will be inlined in specific posts is currently unsupported. Once that becomes supported, this issue will be resolved.