Advance logic for `--base` option

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](https://github.com/troessner/reek/tree/v3.3.0) (2015-08-22)
>
> [Full Changelog](https://github.com/troessner/reek/compare/v3.2.1...v3.3.0)
> **Fixed bugs:**
> - Rake release doesn't work anymore with latest bundler [#643](https://github.com/troessner/reek/issues/643)
> - Rake task blows up with ‘Error: No such file - lib/**/*.rb’ [#642](https://github.com/troessner/reek/issues/642)
> - Make AppConfiguration's initializer flexible enough to deal with different inputs [#608](https://github.com/troessner/reek/issues/608)
> **Closed issues:**
> - private method `load' called for Psych:Module [#653](https://github.com/troessner/reek/issues/653)
> **Merged pull requests:**
> - Remove spec/gem directory. [#659](https://github.com/troessner/reek/pull/659) ([troessner](https://github.com/troessner))
> - Ensure each file can be individually required [#658](https://github.com/troessner/reek/pull/658) ([mvz](https://github.com/mvz))
> - etc...

---

In the end, I want to get something like this as output:
## [v3.3.0](https://github.com/troessner/reek/tree/v3.3.0) (2015-08-22)

[Full Changelog](https://github.com/troessner/reek/compare/v3.2.1...v3.3.0)
- (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](https://github.com/troessner/reek/issues/643)
- Rake task blows up with ‘Error: No such file - lib/**/*.rb’ [#642](https://github.com/troessner/reek/issues/642)
- Make AppConfiguration's initializer flexible enough to deal with different inputs [#608](https://github.com/troessner/reek/issues/608)

**Closed issues:**
- private method `load' called for Psych:Module [#653](https://github.com/troessner/reek/issues/653)

**Merged pull requests:**
- Remove spec/gem directory. [#659](https://github.com/troessner/reek/pull/659) ([troessner](https://github.com/troessner))
- Ensure each file can be individually required [#658](https://github.com/troessner/reek/pull/658) ([mvz](https://github.com/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).