Do not highlight option names in rhs of = in :set

Example, where first occurrence of "trash" should be highlighted,
but the second one should not:

    set trash trashdir=$HOME/.vifm/trash
Highlight numbers in expressions

E.g. in:

    if $VAR < 10
    endif
Highlight wrapped :! commands properly

Example:

nnoremap k :!echo Hello
          \ && echo World<cr>
Highlight all operators in expressions

Most of them were missing from syntax file.
Fix confusing comments in syntax file
Extract prefix commands to a separate group
Make check for <cr> in region case insensitive
Handling of command arguments hi-ting improvements
Merge branch 'improve-syn-hi-a-bit'

Various syntax highlight improvements related to handling of command
arguments.