Add autocomplete to REPL about ferret

[3timeslazy]

@ziflex we can extract it from this

[SumLare]

I want to help with this one, not sure where to look up for registered functions

[ziflex]

@SumLare great!
At this moment, there is no such functionality, you need to add one, something like this to the compiler:

func (c *Compiler) RegisteredFunctions() []string {
	res := make([]string, 0, len(c.functions))

	for name := range c.functions {
		res = append(res, name)
	}

	return res
}

[ziflex]

With DSL syntax itself it's more tricky. You need to take a look at the parser and figure out how to get lexer's tokens.

[ziflex]

Closed #219