Add experimental support of diagram generation through a pipe

[albenik]

Added the ability of generating diagram through plantuml -pipe command without using temporary diagram source files generation.

Which allow us to use such kind of plantuml code with relative imports paths.

@startuml
!include diagram.puml
@enduml

Due to lack of tests to cover the edge cases of added implementation I hid this functional under the feature flag called exp-cmdline-pipe. Then enabled it's replaces the original shell command implementation with the new one.

[sytsereitsma]

Never looked further than the regular PlantUML commands, thanks. This is quite a nice feature, especially when starting plantuml only once and feeding it new images. I will need to investigate this further to see if the pipe option can be leveraged a bit further.

[sytsereitsma]

@albenik I was midway another refactoring and incorporated your work/idea in that.