Added possibility to specify custom popup size

[artemtool]

When we want to use popup.authorize method it 's impossible to customize popup size.

this.auth0.popup.authorize({
            connection: 'facebook',
            redirectUri: 'any',
            popupOptions: {
                width: 200,
                height: 200
            }
        }

[codecov-io]

Codecov Report

Merging #379 into master will decrease coverage by -0.09%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
- Coverage   97.62%   97.54%   -0.09%     
==========================================
  Files          35       35              
  Lines        1177     1179       +2     
  Branches      197      198       +1     
==========================================
+ Hits         1149     1150       +1     
- Misses         28       29       +1

Impacted Files	Coverage Δ
src/web-auth/popup.js	92.85% <50%> (-0.9%)	❌

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 82e5de1...f0e96d4. Read the comment docs.