Added support for overriding column length: Resolves issue 173

[tom-walford]

This is an MR adding support for overriding the terminal column width when rendering ASCII graphs (e.g. dependencyTree)

This uses the inbuilt sbt 1.0 functionality of asciiGraphWidth to set this value (ensuring consistency with the sbt tasks that also render ascii graphs). There is backported support, which depends on an environment variable SBT_ASCII_GRAPH_WIDTH instead (to avoid complex setting requirements).

[cho0o0]

@jrudolph Hi, any plan to add this feature? Actually, I am eager to use it in my project.