Add "elif" statement

Mars does not have any "else if" notion, which makes chained conditional blocks very hard to read. Obviously, C-syntax languages do not have an "else if" either; it is just implicit in the syntax:

if (...)
{
    ...
}
else if (...)
{
    ...
}
else
{
    ...
}

But in Python-syntax languages, such as Mars, this is not so natural, as it requires extra indentation for each "else if" clause:

if ...:
    ...
else:
    if ...:
        ...
    else:
        ...

Therefore, add an "elif" keyword with the same meaning as Python:

if ...:
    ...
elif ...:
    ...
else:
    ...