RFE: support Pandoc HTML template syntax

[sphakka]

Pandoc uses yet another HTML template syntax. No idea if that's based on any know engine. In a nutshell, two syntax variants are possible:

$if(variable)$
This is $variable$
$endif$

or (my preferred one):

${if(variable)}
This is ${variable}
${endif}

Could web-mode support it? It would be awesome :-)