Add no-multiple-var rule

[YuichiNukiyama]

related issue #525.

• for another linter compatibility, I define target options (var,
  let and const)
• I check for statement, but don't check for...in statement and
  for...of statement. becase, TS disallow multiple variable definitions
  in the same statement by default

[jkillian]

Thanks for the PR @YuichiNukiyama!

for another linter compatibility, I define target options (var, let and const)

I'm not sure I understand why we need these options. I think this rule could maybe be simplified and not have any options? What do you think?

[YuichiNukiyama]

@jkillian If you prefer simplicity to compatibility, I remove options (use only true option) and validate all of them (var, let and const) together.

[jkillian]

@YuichiNukiyama No options seems best to me in this case. In what scenarios do you think people might want the different options?

[YuichiNukiyama]

@jkillian Speaking honestly, I don't want these options. So, I follow your idea.

[jkillian]

Looks a lot better, thanks @YuichiNukiyama! One more idea, we might want an "ignore-for-loop" option to make the rule not check for loop declarations. What do you think? Sorry I didn't mention this before!

Edit: Actually, let's do this in a follow-up PR if necessary

[YuichiNukiyama]

@jkillian ignore-for-loop option is good idea. I' ll send new issue and PR.