ENH: Add callable rng for simulation forecasts

[bashtage]

Add callable rng for simulation forecasts so that users can
provide a different rng than the one used in the model.

closes #225

[coveralls]

Coverage increased (+0.8%) to 98.986% when pulling 353f89f on add-rng-forecast into 475ef9a on master.

[codecov]

Codecov Report

Merging #227 into master will decrease coverage by 0.8%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #227      +/-   ##
==========================================
- Coverage   98.19%   97.38%   -0.81%     
==========================================
  Files          30       30              
  Lines        8730     8750      +20     
  Branches      780      751      -29     
==========================================
- Hits         8572     8521      -51     
- Misses         85      156      +71     
  Partials       73       73

Impacted Files	Coverage Δ
arch/univariate/base.py	85.99% <100%> (-10.11%)	⬇️
arch/univariate/mean.py	93.63% <100%> (+0.01%)	⬆️
arch/tests/univariate/test_variance_forecasting.py	99.65% <100%> (ø)	⬆️
arch/tests/univariate/test_mean.py	97.45% <0%> (-1.99%)	⬇️

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