Added `sed ` and `grep` examples/descriptions to useful-commands

Added sed and grep one-liners which I tend to find really useful (would have added a basic awk example but abstained due to being too similar to `cut` example above).

```
+# replaces every occurance of 'apples' with 'oranges' in file.txt
+sed -i 's/apples/oranges/g' file.txt
+# prints the number of lines with the string "foo" in file.txt
+grep -c "foo" file.txt
```