Ignore commonly unwanted files when building XPI

[kumar303]

Fixes #85

[rpl]

This PR looks great!

The only thing that I'm not completely sure about is related to the filesToIgnore defaults and how its overriding will work (especially when it will be available through a config file or a command line option), and as this feature is not exposed to the user yet, most of this could be discussed and resolved in follow-ups if we prefer.

here are a couple of doubts/ideas:

• by looking into the test cases I though that manifest.json is not a great example of possible filesToIgnore customization, because we probably want to be sure it is never excluded by mistake (e.g. by always prepending "!manifest.json" to this array of patterns) or the resulting xpi could be invalid even if the extension source dir was completely valid (excluding the hypotetic filesToIgnore customization).
• how about excluding any hidden file by default? (e.g. by using "**/.*" instead of "**/.git"
• if we suppose that the addon developer can customize the excluded/included patterns somehow, how about composing the default patterns and the user patterns instead of a providing a full override by default? e.g. something like [].concat(mandatory_patterns, user_patterns, defaults_patterns)

[kumar303]

Good points! I fixed the first two. As for concatenating filter overrides, I left it as is. I think this will become more clear once we implement customizations -- I don't want to make it too flexible before then. The feature I added to override all default filter rules will probably still be necessary since it is valid to include nested XPIs inside an XPI. Some very large add-ons do this.

[coveralls]

Coverage increased (+0.05%) to 98.947% when pulling 9ce77c4 on kumar303:build-filter into 01ed902 on mozilla:master.