Allow user to store token from CLI

User story: I am a Liberty contributor. I want to set my GitLab token, so that I can use Bosun to generate reports from GitLab.

Suggested solution: A set-token action that stores the token in the config file, creating it if it doesn't already exist.

$ bosun set-token
Visit https://source.puri.sm/profile/personal_access_tokens to generate
a token with API permissions. Then call `bosun set-token abcdef123` to
store it locally.

$ bosun set-token abcdef123
Your new token has been stored in /home/user/.config/ldh_project_tool/key.yml

No need to handle an existing token, just overwrite the file/value.