crawler-go

TODO

1. rule handling
2. do not insert to queue if already exists
3. page encoding handling
   1. handle unrecognized character
   2. encoding is considered the same accross a group, consider change this
4. front-end rendering
5. conditional rules
6. logging
7. refactoring
   1. manage exported/unexported symbols
   2. namings
   3. create necessary folders
8. retry
9. reports
10. grab text from page
11. filters
12. post actions
    1. make Task.Execute() return a Result object, store the Result into group context
13. Pause X seconds