Add "else if" support in if statements

[yuelinxin]

Is your feature request related to a problem? Please describe.
Add an "else if" statement to the if statement syntax to support a more flexible syntax for code branching & control flow.

Describe the solution you'd like
A typical syntax for "else if" should be sufficient at the moment.
For example:

if condition_1 {
    // branch block
}
else if condition_2 {
    // branch block
}
else {
    // branch block
}