go.mod: set minimum Go version to 1.16 and update module share to v0.25.1

The latest update on share v0.25.1 remove the last boolean parameter
on lib/test.Assert().
Release asciidoctor-go v0.1.0 (2021-03-06)

The asciidoctor-go is the Go module to parse the AsciiDoc (TM) markup
and convert it into HTML5.

This first release bring almost all AsciiDoc syntax except for include
directive, inter-document cross-reference, macros, and non-primary syntax

I hope this library can be useful for Gophers who need the power of
AsciiDoc in their workflows.
all: update dependencies to latest version

While at it, increase the minimum Go version from 1.13 to 1.15
document_parser: remove unused debug statement
all: replace space surrounding the em-dash with html symbol thin space

If the em-dash strings "--" flanked by space, replace the space with
HTML symbol thin space.
inline_parser: add zero-width space after ellipsis character

This is to make it compatible with generated HTML from asciidoctor.
all: add predefined attributes for character replacements
parser: wrap value of email ref name with "mailto:[<value>]"

This is to allow the inline parser to pick it up as macro link.
all: remove parameter isForToC on method element.toHTML

This is to minimize call stack size since the method will be called
document: rename ToEmbeddedHTML to ToHTMLEmbedded

This is for consistency on exported method names on Document object.

While at it, simplify the documentation for exported methods.
document: group exported fields for document readibility
all: move some list ordered class names to constants
all: rename file adoc_table to element_table
all: rename type adocTable to elementTable
README: add subsection for list of TODO for Include Directive
Makefile: add task to run test
all: rename the file adoc_node to element
all: rename adocNode to element

Using prefix adoc to module asciidoc is kinda bit stutter.
all: add unit test for paragraph lead style

Also, mentions in the README that the parser support paragraph lead
style as described in section 18.3 User's manual.
all: do not wrap document with content and preamble on ToEmbeddedHTML

Previously, HTMl document generated from ToEmbeddedHTML contains div
with id "content" and sub div "preamble".

This changes, make the ToEmbeddedHTML generate the HTML document without
those divs wrapper.