Improve / Refactor table format customization API

[phsym]

Table format customization API is a bit weird and not clean. Some refactoring should be done on it.
Builder Pattern may be use in a type named for example FormatBuilder.

What should be customizable :

• Padding
  • Right
  • Left
• Vertical separators
  • Right
  • Left
  • Intern
• Horizontal separators
  • Top
  • Bottom
  • Intern
• Junction characters
• Colspan (delayed to a next release)
• Default style (alignment, colors , ...) (delayed to a next release)
• Implement builder pattern for API
• Change default format constants to functions or lazy_static constants
• Add a section about formatting in README
• Add an example file

[hoodie]

Hi, just since you're openly thinking about it. It would be awesome if the api could allow different kinds of symbols for Top, Bottom and Intermittent separators, or colspans.
What do you think?

This could allow rendering something like this:

┌───────────────────────────────────────┐
│              test table               │
├───────────────────────────────────────┤
│         one  two  three   four   five │
├───────────────────────────────────────┤
│         one  two  three               │
│                3    4     5           │
│           2    3    4     5           │
│           3    3    5     605.0       │
│ hello world                           │
│          27                           │
│           1  two  three               │
└───────────────────────────────────────┘

[phsym]

👍 I'll take a look at this

[phsym]

Colspans and Default styles will be done later and added to a minor release. I want to release v0.6.0 quickly to fix the Type Ascription issue in nightly rust.