feat(gravatar): support letter avatar with weavatar

[devhaozi]

Closes #874

1. 默认使用 WeAvatar 作为头像源（Cravatar 不支持 sha256，WeAvatar 支持 Cravatar 的所有功能）
2. 默认头像采用首字母（由 WeAvatar 提供）
3. 优化了部分文档描述

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.50%. Comparing base (161ff8b) to head (b1c9add).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #882   +/-   ##
=======================================
  Coverage   29.50%   29.50%           
=======================================
  Files         203      203           
  Lines        8575     8575           
=======================================
  Hits         2530     2530           
  Misses       5905     5905           
  Partials      140      140           

Flag	Coverage Δ
go	29.50% <ø> (ø)

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[qwqcode]

为了提高兼容性，老用户可能使用不支持 SHA256 加密算法的 Gravatar API Mirror，我重开了一个 PR 同时适配 MD5 和 SHA256，感谢你的 PR 和提议。

关联：#912