Fix issue #117 Specify base go.mod for app creation

[yxuco]

• Implement fix for #117
• when build with replace in go.mod, check only lines that start with replace
• keep shim source files after shim build

[lixingwang]

@yxuco This feature only for specifying the version of flow and core right? We do have an option --cv to specify the core version. what exactly the issue or use case you want to achieve here? by exposing go.mod file which too technical for the user I think

[yxuco]

--cv is not flexible enough to override other things. For example, I need to build package with shim trigger that requires patches for both flogo/core and flogo/flow versions, as well as changing the module name of the resulting package. It would be simpler to allow a startup mod file that overrides all these attributes, instead of providing multiple flags to override individual attributes such as --cv.

[lixingwang]

@mellistibco thoughts？