ENH: Add variance targetting GARCH model

[bashtage]

Add a VT GARCH model
Note that inference isn't correct

[codecov]

Codecov Report

Merging #250 into master will decrease coverage by 0.67%.
The diff coverage is 15%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
- Coverage    99.4%   98.72%   -0.68%     
==========================================
  Files          31       31              
  Lines        9910     9989      +79     
  Branches      864      871       +7     
==========================================
+ Hits         9851     9862      +11     
- Misses         16       84      +68     
  Partials       43       43

Impacted Files	Coverage Δ
arch/univariate/recursions.pyx	98.48% <ø> (ø)	⬆️
arch/bootstrap/_samplers.pyx	100% <ø> (ø)	⬆️
arch/univariate/volatility.py	94.6% <15%> (-4.55%)	⬇️

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[coveralls]

Coverage decreased (-0.7%) to 99.141% when pulling 1de062e on variance-targetting into a1a73a6 on master.