add 'src search' command for using Sourcegraph search from the command line

Created by: slimsag

This adds a 'src search' subcommand for two primary use cases:

1. Searching from the command line (the default, src search 'some query').
2. Getting results from our search API without having to specify all the GraphQL fields (via the json flag, src search -json 'some query').

For usage information, consult src search -h:

Usage of 'src search':
  -explain-json
    	Explain the JSON output schema and exit.
  -get-curl
    	Print the curl command for executing this query and exit (WARNING: includes printing your access token!)
  -json
    	Whether or not to output results as JSON
  -less
    	Pipe output to 'less -R' (only if stdout is terminal) (default true)

Examples:

  Perform a search and get results:

    	$ src search 'repogroup:sample error'

  Perform a search and get results as JSON:

    	$ src search -json 'repogroup:sample error'

Other tips:

  Make 'type:diff' searches have colored diffs by installing https://colordiff.org
    - Ubuntu/Debian: $ sudo apt-get install colordiff
    - Mac OS:        $ brew install colordiff

  Disable color output by setting NO_COLOR=t (see https://no-color.org).

  Force color output on (not on by default when piped to other programs) by setting COLOR=t

  Query syntax: https://about.sourcegraph.com/docs/search/query-syntax/

Additionally, it also:

1. Cleans up some existing code.
2. Updates our README to accurately reflect the features this tool has.
3. Fixes #5 (closed) by no longer requiring access tokens except on instances that require them, and adding 'just in time' documentation for the user.