Add config option for default GitHub user/organization

[spenserblack]

This adds a config value in a file called default_owner. The config value is in the format name:type, where name is the user or organization, and type is either user or org. octotui will attempt to read from this config file if the search subcommand is not used. If the config value is not set, then the user is instructed to either fill in this config value or use the search subcommand.

Some other changes this PR makes:

• Refactors code so that most of the logic from LoadToken is moved into a more generic function loadConfigFile
• Fixes bug where octotui would fail early if the config folder exists, but not the config file
• Made renderUser and renderOrganization public functions

Resolves #5

[irevenko]

hit me up in Discord: lya Revenko#4742

[spenserblack]

Cleaning up old PRs and forks. Will probably delete the fork soon, but the refs in this PR should still be reachable via refs/pull/NUMBER/head.