Add cfg-field to control default filter inversion

And use it for setting filter.
Update set_view_filter() function in commands.c

 * Move it below places of its use.
 * Add a forward declaration for it.
 * Add leading comment to the function.
 * Change '*' to '[]' in argument list.
Add "f" flag to 'cpoptions' option

It controls default filter inversion for :filter command.
Document "f" flag of the 'cpoptions' option

It controls initial state of filename filter inversion for
:filter[!] {regex} command (in other words, it inverts meaning of
exclamation mark).
Add helptags for 'cpoptions' flags in Vim's docs

 * vifm-cpo-f
 * vifm-cpo-s
 * vifm-cpo-t
Merge branch 'filter-inversion-cpoption'
Treat empty argument to :filter in special way

Which means using of the last search pattern if any.
Document using last search pattern for :filter

For this one should pass an empty argument to the :filter command.
Merge branch 'filter-by-last-search-pattern'