Support discrete-discrete MI

[polsys]

This would be for completeness: no need to use another library for this use-case. Performance shouldn't be a first priority here, it can be improved later.

• [x] Implement naive algorithm for discrete-discrete MI
• [x] Implement naive algorithm for conditional d-d MI
• [x] Implement discrete entropy estimation (with integration tests)
• [x] Add discrete_x parameter to estimate_mi
• [x] Add discrete parameter (array) to pairwise_mi
• [x] Validate that lags/normalization etc. still work
• [x] Extend warnings for continuous data to new methods
• [x] Add some simple benchmark
• [x] Update feature list in docs

Related: #75 (maybe add another section to docs).