work on HTML document including local file
minor fix

* update comments
enclose function in where it is called
don't test cache with empty value
make module for RSS feed
remove redundant wrapper of content document
improve custom URI object handler
define exported function outside for readability
make NG list for text not navigation-like
cache data for the same URL
arrange code for readability

* update comments
* rename a function to clearer name
* expand ternary operators
reduce export functions
add TODO comment