Features for v3

This issue is to track feature implementation for v3. I'm mostly happy with the core functionality in v2, but there are a few rough edges. To that end, v3 will be mostly cleanup and streamlining.
- [x] Make hooks less ugly: `Action(*Context)` should be something like `Action(value Value)` perhaps, and called something other than Action. Not sure. See #125.
- [x] Clean up how boolean flags work (see #54).
- [x] Convert more values to use the generator.
- [x] Some way to manipulate flags after configuration but before parsing. This is useful when a package has defaults that you'd like to override in the main binary. This is currently impossible. (implemented in 292d4f20095c70c7599634c58870b0567d6a6820)
- [ ] Help "sections". This would be used to add sections to man pages, and to the plain text output.
- [ ] Rethink the way `@` and `-` work. See #65.
- [x] Make usage functions more consistently follow a pattern. Similar to how the `regexp` package works.
- [ ] Support a single environment variable that is parsed for flags. eg. `APP="-a -bfoo --something"`
- [x] Configurable help. Some people want enums displayed, default values, envars, etc. Maybe help strings should have their own var expansion.
- [ ] Allow recursive value expansion: eg. given flag `foo=${BAR}` and flag `bar=asdf`, `foo` will evaluate to `asdf`.
- [x] Cleaner usage/help API. eg. `Usage(text)` and `WriteUsage*`.
- [x] Usage templates should allow custom template functions/context.
- [ ] Support [mutually exclusive groups](https://docs.python.org/3/library/argparse.html#mutual-exclusion).
- [ ] Support for mixed mode boolean+value flags. eg. `--purge`, `--purge=24h`? Could use a nil value to indicate not provided? Not sure this is a good idea, plus it's hard to parse.
- [x] Use comma (or semi-colon) separated environment variables, with support for escaping (fixed by resolvers added in b62665b4ea4ea556c9a1ed1525ee77e8e289a9ef).
- [ ] "Fix" default values (see #115).
- [x] Values that open files result in leaked FDs if they have default values (removed feature).
- [x] Ability to customise `--help` flag (see #119)
- [x] `--version` and `--help` should go to stdout, errors to stderr (eg. #120) (implemented in 9f68dc0167caf0e2b2765183636f38d4780e9d44)
- [x] Make TCP types et al return strings rather than `net.*` types.
- [ ] Allow values to be configurable somehow (1)
- [x] Make `Default()` apply before parsing (implemented in 3f9c38e642472881fff9512eae289b40ac4148e1)
- [x] Support flag definitions from tagged structs (implemented in 319ba9076ccd5d047ba10d27a84922a393fbe76d).
- [x] Internationalisation support (started in 20700aa670b125872f58acd318986e777a103ab7)
- [x] Provide a list of environment variables to use as fallback (see #92) (achievable with custom resolvers).
- [x] Some values, such as repeating ones, would benefit from being more configurable. eg. Making sequences optionally support being comma-separated lists. I'm not sure how this would work in practice, but perhaps use the `XXXOption` configuration closure convention? eg. `Strings(options...ValueOption)` where `type ValueOption func(conf map[string]string) error`? (b88248d8da3f8a9eb99fc83b47a53b295e9fc8ee)
- [x] Support for interspersing arguments and commands (eg. `cmd0 cmd1 `) (801d8e545ccdde3fcaaec3ddafc4827969635aa5).
- [x] Distinct boolean and negatable-boolean flags (bf710ba16f982acf6eca0084acd9cb933151aa45).