Add support for a simple config file

[molawson]

Following on from conversation in #124, I'm taking a pass at what I think is a useful but fairly simple approach to supporting a config file. I'd love to hear any and all feedback on this. I see this as a viable solution in particular but also as a means to have more concrete discussions about the topic generally.

Context

As syntax_tree gains wider usage, it'll be very helpful to allow projects to commit their particular configs within their repo for use by any tool that uses stree instead of having to create wrapper scripts, rely on rake tasks that come with their own overhead, or manually keep settings up to date among all contributors.

Approach

This set of changes takes inspiration from Sorbet to provide users with a simple config file that's also easy to parse and use within the gem. It's a text file that has the exact options you'd pass on the command line, each on their own line. These are parsed and prepended to the arguments array within CLI.run, still allowing for other options to passed from the command line.

Limiting Scope (and Future Extensions)

I decided to restrict this only to the command line options and avoid the source files argument, opting to let other tools pass their own source file from the command line, which is preferable for tools like editor integrations that might interact with a single file at a time. If users want to interact with all of their Ruby files at once, the rake tasks are perfect for providing larger, static patterns of files. And since they use CLI.run as well, they'll pick up options from a .syntax_tree file, if present.

I also opted for only supporting a single .syntax_tree file at the project root. If there's a need for multiple configs or config locations, this can be easily extended to look up through a directory structure or accept an option for a specific config file location (or even a different filename). Those felt out of scope for this initial support.

[kddnewton]

Looking at the code, I'm actually more amenable to this than I was. It's pretty clean, and I'm less worried about maintaining it than I was.

A couple of small tweaks and I'll merge this:

• Can you change the name of the file to .streerc? I'm hesitant to use .syntax_tree because that makes me think it might be used at runtime even if the CLI isn't used. Naming it after the executable makes me think it's for configuring the CLI.
• Can you put the name of the file into a constant and then reference that constant in the tests? That'll make it a little more maintainable if we change it in the future.

[molawson]

Thanks for taking a look so quickly! I'm happy to make both of those changes.

[kddnewton]

Thanks!