Add CORS support to MiniProfiler

[pmccloghrylaing]

• Add CorsOrigins property to MiniProfilerOptions
• Set header Access-Control-Expose-Headers: X-MiniProfiler-Ids for profiled cross-origin requests
• Set CORS header Access-Control-Allow-Origin for cross-origin requests handled by MiniProfiler
• Set CORS headers Access-Control-Allow-Headers & Access-Control-Allow-Methods and return 200 for cross-origin OPTIONS requests handled by MiniProfiler.

Closes #502