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]

Codecov Report

❌ Patch coverage is 98.57143% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 86.50%. Comparing base (cf7dca3) to head (083063e).
⚠️ Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
src/tree.rs	98.57%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   86.32%   86.50%   +0.18%     
==========================================
  Files           6        6              
  Lines        1389     1393       +4     
==========================================
+ Hits         1199     1205       +6     
+ Misses        190      188       -2     

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