Fix #20 / Use .IsHome and change way to detect active menu link

[key-amb]

1. Thanks to @ethanmad , use .IsHome introduced in Hugo v0.15 to detect if current page is Home or not
   • See $isHomePage broken by Hugo 1.8 #20
2. Change not to append trailing / by header.html
   • One should add / to link param of menu item if ze wants to match xxx/ URL like below:

# config.toml
[[params.mainMenu]]
  name = "Index"
  link = "index/"

Note

• I do 2nd change because I've noticed the link of Index of demo site http://key-amb.github.io/bootie-docs-demo/ has broken.
• Another way to shoot $isHomePage broken by Hugo 1.8 #20 is here: 0e9a6bc
  • In this change, one have to change the title param of _index.md. I think this is rather confusing, so I don't take this approach.

[ethanmad]

It's strange that change 2 was necessary for you and not for me.

[key-amb]

Maybe the 2nd change is a workaround.
But I can't leave the demo site broken.

Maybe we can do this better by Hugo's menu feature https://gohugo.io/extras/menus/ .

But I've not worked for this yet.

[ethanmad]

I think using the built-in menu feature is a good idea for a future change.
I didn't know about it until just now.