[RFE] Better params ergonomics

Nobody has claimed this yet.

A typical command is currently:

```ruby
AwesomeSpawn.run!("ls", :params => [:a, :l])
```

Most times, params are needed (because sanitizing them is the point of using AwesomeSpawn, otherwise you'd probably just use backticks). But that params key is clunky.

Additionally, the params key is _not_ passed to AwesomeSpawn.build_command_line

```ruby
AwesomeSpawn.build_command_line("ls", [:a, :l])
# => "ls -a -l"
```

As such, I propose we drop the params key altogether, and make it part of the "command" portion via Array or Hash. So, I could see one or all of

```ruby
AwesomeSpawn.run!("ls" => [:a, :l])
AwesomeSpawn.run!(["ls", [:a, :l]])
AwesomeSpawn.run!("ls", [:a, :l])
```

I kind of like the first one, and the second one is just an associative array version of the first (i.e. it's the same as `.to_a`), so it's straightfoward to implement.

Then, we should make `build_command_line` have the exact same interface.

Start by tracing the existing interfaces for AwesomeSpawn.run! and AwesomeSpawn.build_command_line, including how the current params option is handled. Resolve which command/params form is supported and make both entry points share it, with command construction and parameter passing behaving consistently.

Written by the indexing model from the issue text.