Parse a configuration file

[wilzbach]

I really like the installer, but I would prefer if it would use a different directory by default.
This PR allows the installer script to read a configuration file and thus have permanent configuration settings. I know that I could write a small wrapper around the installer or simply patch it, but I believe that I am not the only one having this problem.

tl;dr: With this PR, one can define ~/.config/dlang/installer.cfg and do sth. like:

ROOT=~/.dlang

To use a different default directory.

[dlang-bot]

Thanks for your pull request, @wilzbach!

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[wilzbach]

This needs to be done after the configuration and CLI arguments are read.
Note that at the moment even if --path was set, the temporary directory was ~/dlang/.installer_tmp.

[codecov-io]

Codecov Report

Merging #274 into master will decrease coverage by 0.2%.
The diff coverage is 56.41%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
- Coverage   69.81%   69.61%   -0.21%     
==========================================
  Files           1        1              
  Lines         328      362      +34     
==========================================
+ Hits          229      252      +23     
- Misses         99      110      +11

Impacted Files	Coverage Δ
script/install.sh	69.61% <56.41%> (-0.21%)	⬇️

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[MartinNowak]

Interesting, what other config options come to mind?
We should definitely fix the --path installation.

We could infer the install path from the install.sh location, seems ergonomically better than a config.

[wilzbach]

Interesting, what other config options come to mind?

force-gpg maybe?
no-auto-update (If you want to to do auto-updates of the installer, this would be an option too.

If we start to offer backup mirrors for the downloads, that could be a config as well.

We could infer the install path from the install.sh location, seems ergonomically better than a config.

Well there are use cases where someone wants to use a different download folder for the binaries than the location of the script, but yes I think using the script's directory would be a very reasonable default.

[MartinNowak]

A different path could easily be configured by symlinking ~/dlang to wherever you want.
I'd certainly say that ~/my/path/install.sh should default to use ~/my/path.
The rest of the config options seem vague, so a config file adds complexity and seems unwarranted atm.

[wilzbach]

A different path could easily be configured by symlinking ~/dlang to wherever you want.
I'd certainly say that ~/my/path/install.sh should default to use ~/my/path.

We can't do that without breaking changes.
Currently the location is always:

ROOT=~/dlang

Who knows how people depend on this?

The rest of the config options seem vague, so a config file adds complexity and seems unwarranted atm.

Always updating the installer (#302) is something you requested yourself ;-)
And there we should give people an easy way to opt out. Of course, environment variables would do the job too, but they don't scale well.