Include option to 'manually exclude' files

Provides capability to pass an array of files to be excluded from the
command-line or to be listed as an option in the task file:
blanket_mocha: {
…
options: {
excludedFiles: [
"./src/my/file1.js",
"./src/my/project/file2.js"
]
}
}

or, for command-line, use,
--excludedFiles=["./src/my/file1.js", "./src/my/project/file2.js"]

Result:
The files that have been manually excluded will appear as,
SKIP:  …filename...