support if / elsif / else in block options

[truj]

Support new options:

• if <condition>
• elsif <condition>
• else

All these options should work as a block option for nestable blocks.
For convenience reasons, if should also work as a call option or a channel command option. But for elsif and else that does not make sense.

The condition part should support one boolean expression with 2 values and one operator between them. The following operators should be supported:

• equals (e.g. ==)
• not equals (e.g. !=)

Maybe the following operators could be supported as well:

• lower (e.g. <)
• lower or equal (e.g. <=)
• higher (e.g. >)
• higher or equal (e.g. >=)
• defined (e.g. no operator and only one value)

More complicated boolean expressions could be implemented in the future but for now they are not yet needed. E.g. something like $a==$b||$c<5||$d

[truj]

if as a channel command option will not be implemented. Only for blocks and calls.
defined will be implemented as if $x.
undefined will be implemented as if !$x.
Another condition type is in. Example: if $x in 1; 2; 3; 8