Adding git-ignore command, tests, and documentation. (#264)

that would have "path/to/the/.gitignore" which contains "file.txt". Whereas that does work fine, it can be cumbersome to manage a multitude gitignore files (it can get out of hand). If a policy is in place to consolidate this management into the root gitignore, they'd have to run:

$ cd path/to/the
	... work in that deep directory ...
	... realize that file.txt in that directory needs ignoring ...
	... know relative path of both desired ignore file and cwd ...
$ echo path/to/the/file.txt > ../../../.gitignore
	... or with this script ...
$ git ignore path/to/the/file.txt

Another example might be when the code is deeper embeded, like a java library path.

$ cd code/src/com/mylib
	... work under that directory ...
	... realize subclass/helpers/testdata.json needs ignoring ...
 	... need to know the full path of ignore file, cwd, and
		the relative path to root gitignore ...
$ echo code/src/com/mylib/subclass/helpers/testdata.json > ../../../../.gitignore
- or the more intuitive: -
$ git ignore subclass/helpers/testdata.json
- both ending in the same result
$ cat ${REPO_ROOT}/.gitignore
code/src/com/mylib/subclass/helpers/testdata.json

my mistake, my last email should have read:

$ cd path/to/the
 ... work in that deep directory ...
 ... realize that file.txt in that directory needs ignoring ...
 ... know relative path of both desired ignore file and cwd ...
$ echo path/to/the/file.txt > ../../../.gitignore
     ... or with this script ...
--CORRECTION--
$ git ignore file.txt