Release ciigo v0.5.0 (2021-02-21)

=== New features

* all: implement Watch functionality

  The Watch function, watch any changes on asciidoc files on directory
  "dir" recursively and changes on the HTML template file.
  If there is new or modified asciidoc files it will convert them into HTML
  files using HTML template automatically.

  If the HTML template file modified, it will re-convert all asciidoc files.
  If the HTML template file deleted, it will replace them with internal,
  default HTML template.

=== Breaking changes

* all: return error instead of call log.Fatal on non main packages

  The library, non-main packages, should never call Fatal or panic,
  its up to the main package or the caller on how to handle it.

  While at it, fix the returned error to use log prefix and the error
  value, remove the "ciigo: " prefix.

=== Chores

* all: rewrite to use the watcher

  Now that we have the watcher which task are to watch the asciidoc
  files and template files, we can use it in server to minimize duplicate
  code.

  This changes refactoring the htmlGenerator to initialize the HTML
  template from internal or memfs, so the caller did not need to check by
  itself.