Add infix-to-postfix and regex-tree

[pmcarpan]

• Added infix-to-postfix (shunting yard algorithm)
• Added regex tree construction using above in-to-post
• Added basic pretty-print for tree display
• Improved pretty print, see below for sample output

Full completion of #1

Verify and suggest extra features keeping your task in mind @imraniac

Sample output of pretty-print for (a|b)*abc:

.
├── .
│   ├── .
│   │   ├── *
│   │   │   └── |
│   │   │       ├── a
│   │   │       └── b
│   │   └── a
│   └── b
└── c