Introduce kickstart.stream.contains and kickstart.file.contains

The kickstart.stream.contains and kickstart.file.contains wraps the code
for checking for a string.

docs/kickstart/file/contains.md

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+# kickstart.file.contains file_name string_to_search
+Searchs for `string_to_search` on `file_name`.
+Returns 0 if there the file contains the string, and 1 otherwise.
+
+### Example
+
+```bash
+$ kickstart.file.contains /etc/groups wheel && echo yup
+$ kickstart.file.contains /etc/groups banana || echo nope
+```
+

docs/kickstart/stream/contains.md

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+# kickstart.stream.contains string_to_search
+Searchs for `string_to_search` on STDIN
+Returns 0 if there the stream contains the string, and 1 otherwise.
+
+### Example
+
+```bash
+$ echo weee | kickstart.stream.contains wee && echo yup
+$ echo weee | kickstart.stream.contains ahh || echo nope
+```
+