Soma update command

[KSAlpha]

Introducing soma-list.toml support and some edge cases may cause some unexpected and unwanted behaviors like:

• Problem renamed after update (during build)
• Problem removed after update (during build)
• Large local repository (or mounted nfs) is copied every build

By separating update command from build command, we can address this problem.

Expected command

• soma update Updates all repositories
• soma update [repository_name] Updates the specified repository

[Qwaz]

This was handled in #120. The difference between the issue and the actual implementation is as follows.

• Updating all repositories is not implemented. (yet?)
• Large file handling is not addressed.
• If a problem is removed but there exists an image from the problem, update command is rejected.