Use stdin as input and stdout as output files

[haron]

This PR adds the ability to read from standard input and to write to standard output. Example:

> curl -L https://bicycledutch.wordpress.com/2018/02/20/a-common-urban-intersection-in-the-netherlands/ \
| python -m readabilipy -p \
| jq '.title'
"A common urban intersection in the Netherlands"

[jemrobinson]

Minor changes needed to avoid dangling file handles

[haron]

@jemrobinson please take a look at my fix.

[jemrobinson]

@haron : Thanks for this - I've made a small refactor but I think these changes look good. Let me know if you're happy with this and I'll merge.

[haron]

@jemrobinson I'm happy for sure, thank you!