Adding support for themes

Starting with a bootstrap-based theme (next to a minimal one). More to
follow.

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Update ignore file

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Use a more compact name (`hx`) for the HTML escaping utility function.

However, the old one (`htmlspecialchars`) is still available.
Use new escape function in bootstrap theme
Add new theme: foundation
Fix typo in bootstrap theme
Add note about new themes feature to README
Update tests to latest changes