allow (but ignore) a leading "[no-]" before flag name in @synopsis

`no-` prefix on flag name sets it to `false`, but there was no valid way to show this in usage synopsis. Showing this is relevant for flags with a dynamic default value (like `--color` config flag).  This change makes `@synopsis [--[no-]flag]` valid.
mention dev-build in README. fixes #757
first pass at extracting synopsis from longdesc
remove synopsis tag from the rest of the core subcommands
use fwrite() instead of echo, to prevent buffering
remove synopsis tag where not needed
Merge pull request #762 from wp-cli/fix-762

`WP_CLI::confirm()` doesn't flush output
Merge pull request #755 from wp-cli/deprecate-synopsis-tag

Deprecate @synopsis tag?
Merge pull request #754 from smhmic/patch-2

allow (but ignore) a leading "[no-]" before flag name in @synopsis
rename some variables, for clarity.
add test that catches #663
instead of ignoring them, pass config values along with regular assoc args to validate_assoc()
Merge pull request #767 from wp-cli/fix-663

If a subcommand requires a config value to be set, then check it.
Adds show_admin_column
Merge pull request #768 from thisislawatts/patch-1

Adds show_admin_column
remove extra spaces in scaffold templates
Trimmed whitespace on extracted synopsis lines

Fixes #770 on Windows
Refactored end of line trim of synopsis extraction into regex.
Merge pull request #771 from Rarst/synopsis-extract-fix

Trimmed whitespace on extracted synopsis lines
Fix mysql environment issue.