ArgumentException "unrecognized escape sequence" in RewriteOptions.AddApacheModRewrite in case of usage of regex shorthand character classes (like \d)

### Description of the bug
The System.ArgumentException occurs while executing the method AddApacheModRewrite if passed file contains regex rules with shorthand character classes (like \d).

### To Reproduce
``` C#
public static class ApplicationExtensions
{
public static IApplicationBuilder UseUrlRewriter(this IApplicationBuilder builder)
{
using (var fileReader = File.OpenText("apache_rewrite_rules.txt"))
{
var rewriteOptions = new RewriteOptions()
.AddApacheModRewrite(fileReader);

return builder.UseRewriter(rewriteOptions);
}
}
}
```
Where file `apache_rewrite_rules.txt` contains:
```RewriteRule ^/(\d)$ /?num=$1```

Exception message:
```System.ArgumentException: 'parsing '^/(\d)$' - Unrecognized escape sequence \\d.'```
Stack trace:
```
at System.Text.RegularExpressions.RegexParser.ScanCharEscape()
at System.Text.RegularExpressions.RegexParser.Unescape(String input)
at System.Text.RegularExpressions.Regex.Unescape(String str)
at Microsoft.AspNetCore.Rewrite.Internal.ApacheModRewrite.Tokenizer.RemoveQuotesAndEscapeCharacters(IList`1 tokens)
at Microsoft.AspNetCore.Rewrite.Internal.ApacheModRewrite.Tokenizer.Tokenize(String rule)
at Microsoft.AspNetCore.Rewrite.Internal.ApacheModRewrite.FileParser.Parse(TextReader input)
at Microsoft.AspNetCore.Rewrite.ApacheModRewriteOptionsExtensions.AddApacheModRewrite(RewriteOptions options, TextReader reader)
at Middleware.ApplicationExtensions.UseUrlRewriter(IApplicationBuilder builder, Action`1 configure) in D:\workspace\url-rewriter\UrlRewriter.Middleware\ApplicationExtensions.cs:line 17
at UrlRewriter.Tests.ApacheModRewriteTests.<>c.<.ctor>b__1_0(IApplicationBuilder app) in D:\workspace\url-rewriter\UrlRewriter.Tests\ApacheModRewriteTests.cs:line 22
at Microsoft.AspNetCore.Hosting.DelegateStartup.Configure(IApplicationBuilder app)
at Microsoft.AspNetCore.Hosting.Internal.AutoRequestServicesStartupFilter.<>c__DisplayClass0_0.b__0(IApplicationBuilder builder)
at Microsoft.AspNetCore.Hosting.Internal.WebHost.BuildApplication()
```

### Comments
RewriteRule statement implies to use regular expressions, so it's not clear why there is a call of `Regex.Unescape` in `ApacheModRewrite.Tokenizer.RemoveQuotesAndEscapeCharacters`, seems like it's a bug considering that `Regex.Unescape` is unable to convert sequences such as \w, \d or \s, it throws an ArgumentException.