Add gtree

[ddddddO]

Command line tool to generate tree and create directory from Markdown.

• e.g. generate tree

$ gtree template
- gtree
        - cmd
                - gtree
                        - main.go
        - testdata
                - sample1.md
                - sample2.md
        - Makefile
        - tree.go
$ gtree template | gtree output
gtree
├── cmd
│   └── gtree
│       └── main.go
├── testdata
│   ├── sample1.md
│   └── sample2.md
├── Makefile
└── tree.go

• e.g. generate directory

$ gtree template
- gtree
        - cmd
                - gtree
                        - main.go
        - testdata
                - sample1.md
                - sample2.md
        - Makefile
        - tree.go
$ gtree template | gtree mkdir -e .go -e .md -e Makefile
$ tree gtree/
gtree/
├── cmd
│   └── gtree
│       └── main.go
├── Makefile
├── testdata
│   ├── sample1.md
│   └── sample2.md
└── tree.go

3 directories, 5 files

I would be happy to be merged👍

[ddddddO]

@alebcay Hi! I hope you can see this PR!