feat(avatar): add Avatar component

[liuderchi]

What I did

• add <Avatar> component for displaying user avatar
• add svg image for default condition; render /avatar.svg with svgr.macro
• add storybook examples for general case and default case

How to test

1. yarn start
2. checkout Avatar section in localhost:6006

[evenchange4]

Deploy preview for mcs-lite-ui ready!

Built with commit 350de72fa1e4d64b394d7281575cf93267878094

https://deploy-preview-602--mcs-lite-ui.netlify.com

[evenchange4]

Deploy preview for mcslite ready!

Built with commit 350de72fa1e4d64b394d7281575cf93267878094

https://deploy-preview-602--mcslite.netlify.com

[evenchange4]

Deploy preview for mcs-lite-introduction ready!

Built with commit 350de72fa1e4d64b394d7281575cf93267878094

https://deploy-preview-602--mcs-lite-introduction.netlify.com

[codecov[bot]]

Codecov Report

Merging #602 into master will decrease coverage by 0.75%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #602      +/-   ##
==========================================
- Coverage   84.48%   83.73%   -0.76%     
==========================================
  Files         321      325       +4     
  Lines        2662     2686      +24     
  Branches      171      175       +4     
==========================================
  Hits         2249     2249              
- Misses        352      372      +20     
- Partials       61       65       +4

Impacted Files	Coverage Δ
packages/mcs-lite-ui/src/Avatar/Avatar.js	0% <0%> (ø)
packages/mcs-lite-ui/src/Avatar/index.js	0% <0%> (ø)
packages/mcs-lite-ui/src/Avatar/IconAvatar.js	0% <0%> (ø)
...ckages/mcs-lite-ui/src/Avatar/styled-components.js	0% <0%> (ø)

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