api_key_cmd does not support tilde as "HOME" anymore

[stephane-martin]

After updating the plugin, i get the message: "Config 'api_key_cmd' did not return a value when executed"

• the message is not very explicit, had to dig to find out a "file not found" was happening
• in api_key_cmd, replacing ~ with the explicit home path makes the error disappear

No problem with the new behavior, but should be documented somewhere.

Thanks!