Added sbt-idea dependency

Typing 'sbt gen-idea' generates errors like following:

[error] Not a valid command: gen-idea
[error] Not a valid project ID: gen-idea
[error] Expected ':'
[error] Not a valid key: gen-idea
[error] gen-idea
[error]         ^

To fix this, added 'sbt-idea' plug-in settings to 'plugins.sbt' file.