Add command to display platform specific page

[Mark1626]

What does it do?

Implements the missing commands from the node client

• --linux alias to --platform=linux
• --osx alias to --platform=osx
• --sunos alias to --platform=sunos

Why the change?

Requested in #22

How can this be tested?

Compile the new version and run against any new command and check if the output matches with output --platform flag

Where to start code review?

1. In the main function of tldr.c
2. print_usage function in tldr.c

Relevant tickets?

#22

Questions?

• In print_usage I've used the same text which was present in the issue, is this ok?

[Mark1626]

cc\ @Leandros