Improve similar header matching

[fbiville]

That would help users to perform the initial migration.

The idea could be: match the first comment section before the package declaration, if the header to insert is similar enough (for some value of enough), then let's replace it.

[fbiville]

For the immediate client aka projectriff, matching extra commented newlines is enough (and is now done).

[fbiville]

Made the comparison case-insensitive, ignore starting whitespace chars as well as trailing ones and dots

[fbiville]

One example where headache is too sensitive to variations (on the first run): http URL instead of https (real-life example).
Let's say your license header is as follows:

My awesome license - Copyright {{.YearRange}} {{.Author}}

See more: https://mylicense.example.com

And some files have headers similar but with http URLs:

// My awesome license - Copyright 2012-2013 The original author or authors
// 
// See more: http://mylicense.example.com
[...]

Running headache on this file would cause the old header not to be detected and the new header to be appended before the old one.

To solve this issue, here is a trick one can apply. Parameterize the protocol:

My awesome license - Copyright {{.YearRange}} {{.Author}}

See more: {{.Protocol}}://mylicense.example.com

Add the corresponding definition in the JSON configuration:

[...]
  "data": {
    [...]
    "Protocol": "https"
  }

Run headache and you'll see it matches http and replaces it with https!
Why? Because headache is able to match anything where you define placeholders :)

Once everything is normalized, you can remove the parameter, come back to the initial header license template and run headache again just to make sure it works fine.

You do not need to resort to this if headache has run previously and you want to switch from http to https. Changing the template and re-running headache will be sufficient, as headache is able to parse the last used configuration and match headers accordingly. The above piece of advice applies only for first-time execution.

[fbiville]

Current shortcoming: if the new license header contains an extra newline somewhere, that causes similar headers to not be matched at all. Supporting this should not require to change the whole approach (see #50).