Add tree module to be able tree lignes in row

[davidB]

Hi,

This PR allow to generate tree-line for each row, then user can display it into any cell and produce some kind of tree table like:

  | Title 1     | Title 2    |
  +-------------+------------+
  | 1           | A          |
  | ├─ 2        | B          |
  | │  └─ 3     | C          |
  | └─ 5        | Z          |
  | 6           | Z          |
  | 7           | Z          |
  | ├─ 8        | Z          |
  | └─ 9        | Z          |
  +-------------+------------+

I'm open to any change that should be done to better match the current state of the project (naming, examples, doc).

[codecov[bot]]

Codecov Report

Merging #121 into master will increase coverage by 0.63%. The diff coverage is 98.57%.

@@            Coverage Diff            @@
##           master    #121      +/-   ##
=========================================
+ Coverage   85.86%   86.5%   +0.63%     
=========================================
  Files           5       6       +1     
  Lines        1323    1393      +70     
=========================================
+ Hits         1136    1205      +69     
- Misses        187     188       +1

Impacted Files	Coverage Δ
src/lib.rs	87.54% <ø> (ø)	:arrow_up:
src/tree.rs	98.57% <98.57%> (ø)

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