Introduce better expansion syntax for ranges

[EmilyGraceSeville7cf]

Use parenthesis instead of .. for ranges: {int priority: (1, 10)}. Also add an ability to refer the first or the last range border inside expansions like {int priority: (10, $from + 20)}. In other words allow +, -, *, /, % operators and $from, $to variables inside ranges. Note that just one range border can have math expression, not both and it's not possible to refer to anything outside of the current range.

Notes

• parenthesis should not be escaped
• spacing doesn't matter

Examples

• {/?file(1, 4) image} - expect from one to four images
• {int priority: (1, 10)} - expect one priority value which is in [1..10] range

[EmilyGraceSeville7cf]

Probably I should consider incorporating Go templates.