Fix support of tags files without apple_refs

For the :QHelp command, which wasn't updated to handle such help files.

Thanks to Dmitry Frank.
Update email address of the author
Fix regular expression for filtering help tags

Filter string didn't contain enough backslashes needed because of
double quotes.

Thanks to Dmitry Frank.
Fix regular expression for filtering help tags

Filter string didn't contain enough backslashes needed because of
double quotes.

Thanks to Dmitry Frank.
Don't escape g:qthelp_browser on invocation

Now it's possible to specify parameter list along with command to
be executed as well as use whatever escaping user wants.

Thanks to Dmitry Frank.
Add g:qthelp_tags option

Optionally contains custom value for the 'tags' option, which will
be used to look for Qt documentation tags.

Thanks to Dmitry Frank.
Extend first item of using instructions

Specify how to generate tags for Qt documentation.

Thanks to Dmitry Frank.