Highlight :vs[plit] in syntax

Somehow missed this command before.
Use separate id's for :else and :endif commands

Used to share one id with the :if command.
Extract cmd_should_be_processed() function

Decides whether next command with id cmd_id should be processed
or not, taking state of conditional statements into account.
Fix processing of some forms of nested :if stmts
Add unit-tests for :if-:else statements
Merge branch 'fix-nested-if-else-statements'

Fix processing of some forms of nested :if statements.  Used to treat
:else and :endif commands of inner statements as part of outer ones.