Feature/read from stdin

[lmammino]

Allows using - as a token argument to read the value from stdin instead.

Closes #6

Example:

echo "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4geHggRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ.ifqduBDRMJbz7BKGEzhWSZw5P_MxNPPrFHovfEob49M" | ./target/release/jwtinfo -
{"sub":"1234567890","name":"John xx Doe","iat":1516239022}