Expose request timeout of themekit in .env

[Liroo]

This PR expose the request timeout optional setting of themekit in the .env of slate.

A .env at the root such as:

SLATE_STORE=***.myshopify.com
SLATE_PASSWORD=***
SLATE_THEME_ID=***
SLATE_IGNORE_FILES=config/settings_data.json
SLATE_TIMEOUT=2m

will set the timeout in config.yml at 2m SLATE_TIMEOUT used by themekit.

This closes #831, fixes #699 and maybe some others I didn't saw.

Checklist

For contributors:

• I have updated the documentation
• New .env setting exposing themekit timeout

For maintainers:

• No major update

This is my first PR on Slate and maybe I'm not aware of everything in the project. I needed to modify the timeout for my own and saw some issues on this.

[t-kelly]

Great PR. Reviewed and tested. Works as expected. Thanks @Liroo!

[lock]

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