Add labels to connector lines

[roland-KA]

This is the implementation of an additional connector_labels-parameter which allows arbitrary labels on the connector lines of the tree as discussed in #7 .

@ablaom , @sylvaticus: Would this be a suitable solution to what we discussed in #7?

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (a76af19) 98.41% compared to head (4215e41) 92.85%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8 +/- ## ========================================== - Coverage 98.41% 92.85% -5.56% ========================================== Files 1 1 Lines 63 70 +7 ========================================== + Hits 62 65 +3 - Misses 1 5 +4 ``` | [Files](https://app.codecov.io/gh/JuliaAI/TreeRecipe.jl/pull/8?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaAI) | Coverage Δ | | |---|---|---| | [src/TreeRecipe.jl](https://app.codecov.io/gh/JuliaAI/TreeRecipe.jl/pull/8?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaAI#diff-c3JjL1RyZWVSZWNpcGUuamw=) | `92.85% <58.33%> (-5.56%)` | :arrow_down: |

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[roland-KA]

Thanks for the hint with codecov. I've added a basic configuration file.

[ablaom]

Thanks for the addition.