Ending with an optional parameter breaks expression

[nblackburn]

Ending a route binding with an optional parameter like the one below, results in the expression breaking.

const {id, action} = match('/users/:id/:action?', '/users/1/delete');

Stacktrace

C:\Users\nblac\Projects\micro-match\source\index.js:46
    return new RegExp(`^${expression}$`);
           ^

SyntaxError: Invalid regular expression: /^/users/([^/]+)/)?([^/]+)?$/: Unmatched ')'
    at RegExp (native)
    at buildExpression (C:\Users\nblac\Projects\micro-match\source\index.js:46:12)
    at match (C:\Users\nblac\Projects\micro-match\source\index.js:92:22)
    at Object.<anonymous> (C:\Users\nblac\Projects\micro-match\source\index.js:102:22)
    at Module._compile (module.js:556:32)
    at Object.Module._extensions..js (module.js:565:10)
    at Module.load (module.js:473:32)
    at tryModuleLoad (module.js:432:12)
    at Function.Module._load (module.js:424:3)
    at Module.runMain (module.js:590:10)