#1 Implement templating engine

[Shuriken]

A templating engine would allow themes to be more customizable and
flexible. I'm thinking about using Smarty, but if that turns out to degrade
performance too much, I'll look for lighter alternatives or make my own.

What's needed:
- Basic variable parsing
- Variable manipulation functions (dates, sizes, HTML entity
conversion, ...)
- Conditional and looping structures

[Shuriken]

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...Not to mention it'd help clean up the code a lot.