Organize flags

Sorted extension flags in four sections (b224473):

 - **Block-level extension**: generally adds support for a new block-level callback or block type.
 - **Span-level extension**: generally adds support for a new span-level callback or span type.
 - **Other flags**: generally modifies, limiting an existing feature or making it permissive.
 - **Negative flags**: disables an existing feature.

On top of that, also defined some sets of flags (83a932b), corresponding to the first three sections.

Also renamed `HOEDOWN_TABLE_ALIGN_CENTER` to `HOEDOWN_TABLE_ALIGN_C`, for consistency with `HOEDOWN_TABLE_ALIGN_L` and `HOEDOWN_TABLE_ALIGN_C`.

#### Motivation

When the users of Sundown / Hoedown want the extensions (aka GFM) they'd usually write `~0` instead of specifying them one by one. This doesn't seem correct in the long time.

The introduction of `HOEDOWN_EXT_DISABLE_INDENTED_CODE` completely ends this. `~0` no longer enables all features, it also disables some. We need to palliate this.

Now, lazy users can use a combination of `HOEDOWN_EXT_BLOCK`, `HOEDOWN_EXT_SPAN` and `HOEDOWN_EXT_FLAGS` to specify what kinds of features they want without disabling any.