Adding many new features

[nyancol]

This pull request contains:

• [vim] Some groups can get a unique color that won't be used with other elements (e.g.: the color used for the comments can not be used for other syntactic elements)
• [vim] Now vim is able to find the location where a given plugin has been installed. This is used to automatically configure lightline
• [imgur] Adding a simple function that downloads an image from a url
• [filter] The luminosity and saturation filters have been rewritten to be more accurate
• [reduction] A pass has been added before running the algorithm that sets a maximum number (60) for each "cluster" of colors.

Currently, the class that filters the luminosities and saturations is called LumFilter, which is not really appropriate. Any better ideas? I would like to avoid using the term "filter" since it is also a builtin python function. I don't expect you to read all the code added, but at least tests/test_reducer.py and tests/test_lum_filter.py, which reflects more the capabilities of the new filters. To run an individual test: python3 -m unittest tests.test_reducer.TestReducer.test_reducer_taipei

[nyancol]

Two variables that might have to be recalibrated are:

• hapycolor.filters.reducer.Reducer.get_maximum_clique() fixes the largest connected graph to 60, but maybe on some computers it could be too large and so, it would be too slow.
• hapycolor.filters.reducer.Reducer.threshold defines the minimal distance between two colors. To calibrate this value, the easier way would be to uncomments the prints of this test: tests.test_reducer.TestReducer.test_reducer_taipei and run python3 -m unittest tests.test_reducer.TestReducer.test_reducer_taipei

[nyancol]

I tried to improve the coding style in various files. Maybe you had something more specific in mind?