Update gitignore.

[junaruga]

• Add "/" to ignore the top directories.
• Add ignored nvm install directory and katex directory that are created after installing the dependency packages.

For example, currently when creating below file test/pkg/a.txt, the file is ignored unintentionally.

$ mkdir test/pkg/
$ touch test/pkg/a.txt

But after this pull-request, it is appeared.

$ git status
On branch hotfix/gitignore
Untracked files:
  (use "git add <file>..." to include in what will be committed)

	test/pkg/

[gettalong]

I see what you get at but this is currently no problem and I don't think that it will be in the near future.

[junaruga]

I see. How about the directories I added?

node_modules
katex

When you do tests on you local, you see the untracked files by git status don't you?

$ git status
On branch master
Your branch is up to date with 'origin/master'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)

	katex/
	node_modules/

[junaruga]

Ah an another person talked with you about the same topic in another PR.
#518