[jarun#702] implemented fetch-status & export-on operations

[LeXofLeviafan]

fixes #702 and #708:

• implementing a fetch_data() function returning a FetchResult namedtuple including additional information (+deprecating network_handler())
• adding a --url-redirect option for add/update action (url_redirect parameter in API) to update the URL by following permanent redirects
• adding a --tag-redirect option for add/update action (tag_redirect parameter in API) to procure an extra tag based on HTTP code (when it's a permanent redirect); optionally customizing the tag
• adding a --tag-error option for add/update action (tag_error parameter in API) to procure an extra tag based on HTTP code (when it's an error response); optionally customizing the tag
• adding a --del-error option for add/update action (del_error parameter in API) to delete/prevent adding the URL upon receiving an error response; optionally specifying which errors are matched (a network error will always match when adding, but not when updating – to prevent deleting existing bookmarks due to a network failure)
• adding a --export-on option for add/update action (export_on parameter in API) to adjust the output of an --export (filtering export by matched HTTP responses and including replaced URLs/deleted bookmarks in the output); this matches the HTTP codes of other such operations by default, which optionally can be overridden
• documenting the new options in CLI help, README and manpage
• implementing tests to ensure that various combinations of these parameters work as expected, including retained original behaviour of the code without it

also:

• improving testing performance by mocking HTTP/fetch_data() responses
• adding support for multiple index values in update_rec(), append_tag_at_index(), delete_tag_at_index(), refreshdb(), print_rec()

[LeXofLeviafan]

…Whoa. These typings are really hard to appease on multiple Python versions.
(And it seems like tox doesn't even try using different Python versions with this config 😅)

[jarun]

Thank you!