add heatmap font color threshold parameter

[rasbt]

Adds a new parameter to the heatmap function:

text_color_threshold : float (default: None)
    Threshold for the black/white text threshold of the text
    annotation. Default (None) tried to infer a good threshold
    automatically using `np.max(normed_matrix) / 2`.

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Codecov Report

Patch coverage has no change and project coverage change: -0.03 :warning:

Comparison is base (77d0696) 77.30% compared to head (e2f7bc1) 77.28%.

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```diff @@ Coverage Diff @@ ## master #1022 +/- ## ========================================== - Coverage 77.30% 77.28% -0.03% ========================================== Files 200 200 Lines 11311 11314 +3 Branches 1486 1487 +1 ========================================== Hits 8744 8744 - Misses 2348 2351 +3 Partials 219 219 ``` | [Impacted Files](https://codecov.io/gh/rasbt/mlxtend/pull/1022?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka) | Coverage Δ | | |---|---|---| | [mlxtend/plotting/heatmap.py](https://codecov.io/gh/rasbt/mlxtend/pull/1022?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka#diff-bWx4dGVuZC9wbG90dGluZy9oZWF0bWFwLnB5) | `0.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Sebastian+Raschka)

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