feat: include/exclude with text/template filters

This changeset also introduces some new methods on `walk.File`,
including a method for extracting a shebang (if present), a method for
extracting the interpreter portion of the shebang, a method indicating
whether or not the file is executable, and a convenience method that
indicates if the file "looks like" a script.

Author: tomeon

cmd/init/init.toml

@@ -61,6 +61,10 @@ options = []
 includes = [ "*.<language-extension>" ]
 # Glob patterns of files to exclude
 excludes = []
+# Template filters for selecting files
+select = [ "{{ }}" ]
+# Template filters for rejecting files
+reject = [ "{{ }}" ]
 # Controls the order of application when multiple formatters match the same file
 # Lower the number, the higher the precedence
 # Default is 0