It's addition for #258.

For now --base option just concat old change log with new, auto-generated.
It would be nice to use full power of Reader, to not just fetch last tag, but also fetch content for each issue and put auto-generated section for each tag, and then data from content.

For example, for https://github.com/troessner/reek project

For tag 3.3.0:

Form manual change log:

3.3.0 (2015-08-23)

• (HParker / Adam Hess) add support for or-assign constants
• (troessner) Update TooManyMethods default value to 15

For auto-generated change log:

v3.3.0 (2015-08-22)

Full Changelog
Fixed bugs:

• Rake release doesn't work anymore with latest bundler #643
• Rake task blows up with ‘Error: No such file - lib/**/*.rb’ #642
• Make AppConfiguration's initializer flexible enough to deal with different inputs #608
  Closed issues:
• private method `load' called for Psych:Module #653
  Merged pull requests:
• Remove spec/gem directory. #659 (troessner)
• Ensure each file can be individually required #658 (mvz)
• etc...

In the end, I want to get something like this as output:

v3.3.0 (2015-08-22)

Full Changelog

• (HParker / Adam Hess) add support for or-assign constants
• (troessner) Update TooManyMethods default value to 15

[//]: # (START OF AUTO CHANGE LOG CONTENT)

Fixed bugs:

• Rake release doesn't work anymore with latest bundler #643
• Rake task blows up with ‘Error: No such file - lib/**/*.rb’ #642
• Make AppConfiguration's initializer flexible enough to deal with different inputs #608

Closed issues:

• private method `load' called for Psych:Module #653

Merged pull requests:

• Remove spec/gem directory. #659 (troessner)
• Ensure each file can be individually required #658 (mvz)
• etc...

The string [//]: # (START OF AUTO CHANGE LOG CONTENT) will be invisible in markdown file, but helps to detect where manual change log ends (could be helpful in the future).