Feat: add alias for ghq get

[Okabe-Junya]

close #374

Feature Description

• add ghq clone command
  • ghq clone is an alias for ghq get

Example

$ pwd
path/to/ghq (forked)

$ make build && ./ghq clone https://github.com/x-motemen/ghq
     clone https://github.com/x-motemen/ghq -> /Users/junya/src/github.com/x-motemen/ghq
       git clone --recursive https://github.com/x-motemen/ghq /Users/junya/src/github.com/x-motemen/ghq
Cloning into '/Users/junya/src/github.com/x-motemen/ghq'...
remote: Enumerating objects: 3325, done.
remote: Counting objects: 100% (441/441), done.
remote: Compressing objects: 100% (220/220), done.
remote: Total 3325 (delta 243), reused 368 (delta 213), pack-reused 2884
Receiving objects: 100% (3325/3325), 820.82 KiB | 4.80 MiB/s, done.
Resolving deltas: 100% (2024/2024), done.

[Okabe-Junya]

friendly ping @Songmu

I would be happy if you take a look this PR. Thank you :D

[Songmu]

Thanks.
I'm unsure if ghq needs to resemble the Git command system, but since you made it, I'll merge it.

[mlavi]

I appreciate this, I think of ghq in context of git and it is helpful for reuse.