core: add option "-mask" in command /unset (closes #112)
core: fix evaluation of expressions with regex condition (closes #63)

The regex itself is not evaluated any more (so parentheses are kept).

Before the fix:
>> abcd =~ (?-i)^abc
== [0]
>> (abcd) =~ \(abcd\)
== [0]

After the fix:
>> abcd =~ (?-i)^abc
== [1]
>> (abcd) =~ \(abcd\)
== [1]