:bug: Fix undefined appearing after a commit link

When a commit does not have a closes expression,
`undefined` was appearing next to the commit link.
:bug: Fix section randomized order

The first section to be displayed was the one of the latest
commit, and not as set in the config.
:lipstick: Replace section by version every time section don't make sense

There was a confusion in some place between sections (as
defined in a config) and version blocks (where many sections
can appear). This commit normalize the wording across
sources.
Add one letter aliases for all cli options
Implement CLI accepting a version as argument

Calling `changelog {TAG}` will allow to replace the normally
`undefined` tag in the generated markdown by the value of
`{TAG}`.
Prepare 0.2.0 release