Allow custom source directory names, like nativeMain

[petertrr]

Describe the bug

Official guidelines for sharing code among platforms suggest using directories like nativeMain. Diktat now allows only common* and platform-specific names. We should add a new configuration option in cofig.

Environment information

• diktat version: 0.4.0

[petertrr]

We have discussed it previously, but now, digging a little bit more into the world of kotlin multiplatform, I see that people use custom directory names more often than we have thought before: not only nativeMain, but things like mobileMain, commonMobileMain or even nonJvmMain.