Allow for reading theme information from standalone files (cf PR #366)

[PicoJr]

Allow for reading theme information from standalone files (cf PR #366)

This PR contains the original 2 commits from atheriel (rebased).

Thanks to @atheriel who did the heavy-lifting =)

It should be compatible with previous config file format: see unit tests in src/config.rs.

I took the opportunity to move has_command from pacman.rs to utils.rs.

I also added some unit tests for color_from_rgba, I changed the code a little to fix a panic when input is too short: color_from_rgba("rust") would panic...

[atheriel]

This looks good to me in general, thanks for taking up the cause. Two concerns:

1. As decided in Provide a default config file paths #465, we should use i3status-rust for config paths, not i3status-rs.
2. This introduces about a dozen new dependencies to run the unit tests -- is that necessary?

[PicoJr]

This looks good to me in general, thanks for taking up the cause. Two concerns:

1. As decided in #465, we should use `i3status-rust` for config paths, not `i3status-rs`.

Config path fixed in latest commit.

2. This introduces about a dozen new dependencies to run the unit tests -- is that necessary?

I wouldn't say it is necessary: It might be possible to adapt the tests so that no temporary files are required (we wouldn't depend on assert_fs)

I would argue it's only dev-dependencies: assert_fs is not required to build the project, only to run the tests.
Nevertheless it still negatively impacts CI validation time (by how much I don't know).

If this new dependency is an issue don't hesitate to reach me out :)

[ammgws]

I would argue it's only dev-dependencies: assert_fs is not required to build the project, only to run the tests.

I think it's fine if it's just a dev dep. Also it could lead the way for tests to be introduced in the other blocks too. @atheriel ?