This avoids double parsing the page content when `enableEmoji=true`.
This commit also adds some general improvements to the parser, making it in general much faster:
```bash
benchmark old ns/op new ns/op delta
BenchmarkShortcodeLexer-4 90258 101730 +12.71%
BenchmarkParse-4 148940 15037 -89.90%
benchmark old allocs new allocs delta
BenchmarkShortcodeLexer-4 456 700 +53.51%
BenchmarkParse-4 28 33 +17.86%
benchmark old bytes new bytes delta
BenchmarkShortcodeLexer-4 69875 81014 +15.94%
BenchmarkParse-4 8128 8304 +2.17%
```
Running some site benchmarks with Emoji support turned on:
```bash
benchmark old ns/op new ns/op delta
BenchmarkSiteBuilding/TOML,num_langs=3,num_pages=5000,tags_per_page=5,shortcodes,render-4 924556797 818115620 -11.51%
benchmark old allocs new allocs delta
BenchmarkSiteBuilding/TOML,num_langs=3,num_pages=5000,tags_per_page=5,shortcodes,render-4 4112613 4133787 +0.51%
benchmark old bytes new bytes delta
BenchmarkSiteBuilding/TOML,num_langs=3,num_pages=5000,tags_per_page=5,shortcodes,render-4 426982864 424363832 -0.61%
```
Fixes#5534
When the page parser was rewritten in 0.51, this was interpreted literally, but commented out front matter is used in the wild to "hide it from GitHub", e.g:
```
<!--
+++
title = "hello"
+++
-->
```
Fixes#5478
This commit also pulls down the log level for a set of WARN statements to INFO. There should be no ERRORs or WARNINGs in a regular Hugo build. That is the story about the Boy Who Cried Wolf.
Since the WARN log is now more visible, this commit also improves on some of them, most notable the "layout not found", which now would look something like this:
```bash
WARN 2018/11/02 09:02:18 Found no layout for "home", language "en", output format "CSS": create a template below /layouts with one of these filenames: index.en.css.css, home.en.css.css, list.en.css.css, index.css.css, home.css.css, list.css.css, index.en.css, home.en.css, list.en.css, index.css, home.css, list.css, _default/index.en.css.css, _default/home.en.css.css, _default/list.en.css.css, _default/index.css.css, _default/home.css.css, _default/list.css.css, _default/index.en.css, _default/home.en.css, _default/list.en.css, _default/index.css, _default/home.css, _default/list.css
```
Fixes#5203
`*json.UnmarshalTypeError` and `*json.SyntaxError` has a byte `Offset`, so use that.
This commit also reworks/simplifies the errror line matching logic. This also makes the file reading unbuffered, but that should be fine in this error case.
See #5324
