Better support for ternary operator

[wisskid]

Smarty has support for the ternary operator since 2009/2010. But it was never documented. Also, it requires the testing expression to be in parentheses. So this works now:

{($foo) ? $var1 : $var2}

But this will not:

{$foo ? $var1 : $var2}

There is no support for the shorthand ?: version.

• Change the implementation to make the parentheses optional
• Support the shorthand version as well
• Document the implementation