Added Ignore Command

[WillFantom]

Simply added an toggle-ignore command that sets files to be ignored by dropbox or synced via the filesystem extended attributes.

Not only can a file or directory's ignored state be toggled, but also a set of files/directories based on a .gitignore style file (as suggested in issue #125). This can be done by instead of specifying a file to toggle, by proving the gitignore style file to be used along with the -g flag.

Examples:

• Assuming the file example-file.md is within a Dropbox directory (thus has the xattr com.dropbox.attrs) and is currently synced, running the following would set it to be ignored:

  dbxcli toggle-ignore example-file.md

  and running the same again would set it back to being synced.

• Assuming the file .gitignore is in the current directory, and contains just example/ and test.*, running the following would toggle the ignored state of all the files within those defined scopes. e.g. example/a/b, test.a, test.b would be toggled:

  dbxcli toggle-ignore -g .gitignore

[WillFantom]

Also, the issue #157 should be closable with this 👍 However, an important distinction to make is that of syncing from a given point, or syncing at all. This PR implements the latter.

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[bandtank]

adding a flag specifically for git sounds like a good idea, but it has a lot of potential to go wrong if the git ignore file has complex expressions. i would remove that.