Perf: add API to avoid regex matching rewrite rules unless necessary

Regex is an expensive part of the rewrite middleware a lot in order to do pattern matching. It is common for rewrite patterns to be led by a static prefix.

Example:
"blogs/(.*)" => "api/blogs?id=$1"

For this rule, the middleware can avoid unnecessary regex allocations by only running the rule when the path begins with "/blogs/".

Possible API design:

``` c#
RewriteOptions.AddRewrite(string prefix, string pattern, string urlResult) // prefix is static, doesn't container rules. If uri doesn't start with the prefix, skip this rule
RewriteOptions.Ignore(string prefix); // always skip rule processing for urls beginning with this prefix
```

Usage

``` c#
RewriteOptions.AddRewrite(prefix: "/blogs/", pattern: "(.*)", urlResult: "/api/blogs?id=$1")
RewriteOptions.Ignore("/images/")
```