Use const by default

[danielo515]

Hello again,

Thank you very much for this awesome project, I'm really enjoying it.
It would be awesome if the compiled output contains constants whenever possible. Currently you use let by default, which is fine for many cases, but with my style of programming I rarely need to mutate a variable, and when it happens it's usually an error.

There are eslint rules to detect when a variable is never re-assigned and they suggest you to use const, so similar logic could be used.

What do you think ?

Regards