Minor improvements

• add signature to :DUddoc
• Allow disabling format and indent with dfmt
• When dub fails, still get the import path in g:dutyl_stdImportPaths
• Remove dub warnings
• echoerr unparsable dscanner output lines
• Fix a bug with tools that use default executable
• Fix sending input to vimproc#system
• Allow registering complex commands for running tools