Add align_all feature

That's in preparation of clap-rs#1013: provide option to layout the output
of "my_program --help" with the same alignment across sections.

Cargo.toml

@@ -54,6 +54,7 @@ lints       = ["clippy"] # Requires nightly Rust
 debug       = [] # Enables debug messages
 no_cargo    = [] # Enable if you're not using Cargo, disables Cargo-env-var-dependent macros
 doc         = ["yaml"] # All the features which add to documentation
+align_all   = [] # When generating help, align the text across sections (#1013)
 
 [profile.release]
 opt-level = 3