A minimal template parser to use with generated static gemlogs

Gemoire uses its own template system, where the templates are plain texts with special inline components. The valid components are:

• {$pholder$} - A placeholder with the key pholder
• {$pholder:default value$} - A placeholder with a default value
• {&pholder:fallback&} - With a fallback key to use in case pholder has no value
• {&pholder:fallback:default value&} - With a fallback key and a default value
• {# comment! #} - An unrendered comment
• {\ constant \} - An unevaluated block, akin to escaping things inside it

All inline components can be multiline in any way, including placeholder keys. The arguments can also be surrounded by any amount of whitespace, which will get stripped except when specifying a default value.

The surrounding braces are parsed in order, so the components can not be nested.