Add exercises to implement shell tools (#7)

These exercises are language-agnostic - the same description and test
files work for any language. In sprint 3 we will set implementing these
in JS, and in sprint 4 we will set implementing these in Python.

Author: illicitonion

.gitignore

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+node_modules

implement-shell-tools/README.md

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+# Implement shell tools
+
+Your task is to re-implement shell tools you have used.
+
+Each directory contains instructions for one shell tool to re-implement.
+
+In general your success criteria are that the tools give the same output as the real shell tools on your computer.
+
+You will be told in your coursework issue which language you should use in your implementation.
+
+If the language you're using requires an interpreter, it's fine for the commands you run to be prefixed with that. i.e. if you're implementing `cat` in `NodeJS` it's fine if you need to run `node cat.js 1.txt` instead of `cat 1.txt`.

implement-shell-tools/cat/README.md

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+# Implement `cat`
+
+You should already be familiar with the `cat` command line tool.
+
+Your task is to implement your own version of `cat`.
+
+It must act the same as `cat` would, if run from the directory containing this README.md file, for the following command lines:
+
+* `cat sample-files/1.txt`
+* `cat -n sample-files/1.txt`
+* `cat sample-files/*.txt`
+* `cat -n sample-files/*.txt`
+* `cat -b sample-files/3.txt`
+
+Matching any additional behaviours or flags are optional stretch goals.
+
+We recommend you start off supporting no flags, then add support for `-n`, then add support for `-b`.

implement-shell-tools/cat/sample-files/1.txt

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+Once upon a time...

implement-shell-tools/cat/sample-files/2.txt

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+There was a house made of gingerbread.

implement-shell-tools/cat/sample-files/3.txt

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+It looked delicious.
+I was tempted to take a bite of it.
+But this seemed like a bad idea...
+
+There's more to come, though...

implement-shell-tools/ls/README.md

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+# Implement `ls`
+
+You should already be familiar with the `ls` command line tool.
+
+Your task is to implement your own version of `ls`.
+
+It must act the same as `ls` would, if run from the directory containing this README.md file, for the following command lines:
+
+* `ls -1`
+* `ls -1 sample-files`
+* `ls -1 -a sample-files`
+
+Matching any additional behaviours or flags are optional stretch goals.
+
+We recommend you start off supporting just `-1`, then adding support for `-a`.

implement-shell-tools/ls/sample-files/.hidden.txt

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+Once upon a time...

implement-shell-tools/ls/sample-files/1.txt

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+There was a house made of gingerbread.