Improvements for greater flexibility

[seirios]

This PR contains three improvements which I've found useful when using pheatmap.

1. Honor number_format when using a matrix in display_numbers. This is technically a bugfix(?)
2. Allow number_color to be a function, which allows setting text color based on background color. This is useful for getting good contrast against colormaps.
3. Honor gaps_row/col settings when clustering rows/columns if cutree_rows/cols is unset. This allows for greater customization and doesn't hurt, right?

Cheers!