JSON, TOML output for tables

[mosteo]

This is the low hanging fruit for structured output. Commands that use tables for reporting now can output the same information in JSON and TOML formats with a new --format[=JSON] global switch:

$ alr --format search --crates hello
[                                               
  {
    "name": "hello",
    "description": "'Hello, world!' demonstration project"
  },
  {
    "name": "libhello",
    "description": "Basic library demonstration project"
  }
]

$ alr --format=TOML search --crates hello
[[data]]                                        
description = "'Hello, world!' demonstration project"
name = "hello"
[[data]]
description = "Basic library demonstration project"
name = "libhello"

Output is always an array of records (one per row), with headers acting as keys. Since TOML can't have an anonymous top-level array, it is nested within a map with a single data array.

Some commands that use custom output remain unaffected (show, with). Commands that do use tables like this are e.g. search, toolchain, pin, index, version.