This allows importer.py to process Netscape HTML exports from Firefox
(and other Mozilla browsers) with three distinct types:
* bookmarks (sans shortcuturl attribute)
* keywords (bookmarks with a shortcuturl attribute)
* searches (keywords with a URL containing a %s substitution)
The first two can be combined at will in either quickmark or bookmark
output formats, the only difference being that keywords will be used in
place of titles when exporting to quickmark format. Searches are
exported to qutebrowser.conf format, or the new config.py format.
Dictionaries are used in the import function for readability's sake, but
the command line arguments follow the same general formula of true-false
flags used to select input bookmark types and the output format.
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