feat(comment): add gitment

[ElderJames]

What kind of change does this PR introduce? (check one with "x")

• Bug fix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

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Description

Gitment is a comment system based on GitHub Issues, which can be used in the frontend without any server-side implementation. Now I add it in this theme.

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Verification steps

1. Click here to register an OAuth application, and you will get a client ID and a client secret.

Make sure the callback URL is right. Generally it's the origin of your site, like https://shriek.tech.

2. Change the theme settings :

use: gitment
gitment_repo:   # git repo of the hexo
gitment_owner:  # git repo's owner
gitment_client_id:  # github app client id 
gitment_client_secret :  # github app client secret 

3. Screenshot:

[neoFelhz]

Great!