Made it possibly to specify where in the markdown file the summary is placed.

[Kreinoee]

This fixes #5

This is done by adding the options --replace_summary_token [summary_token]. When called with this option, the script replaced the line containing [summary_token] with the summary, instaed of placing it in the top.

Also made some other improvements on the way:

• No shell commands, everything is now done in plain ruby.
• It now only overwrites output files if called with --force or -f
• Its now possibly to specify what heading level the heading of the summary should have with --heading_level [integer], or to disable the summary header by specifying 0.
• It now creates tempfiles in the operating system temp directory, instead of the current dir.