feat: Add cwd option to use relative paths for compiler args

[nandenjin]

Resolves #13

Proposal

• Add cwd as loader option. If specified, change paths that are passed to compilers to relative ones.
• Update README to follow the above change.

Note

I add some conversion codes before executions of utility.execute(), but originally paths are explicitly converted to be absolute at process(), so it may be more simple to make paths to be handled as relative internally.

My implementation intends not to add breaking changes to the original behavior (yes, it should make no changes as long as you don't use the cwd option), but if requested, I'll try another way.

[nandenjin]

Since this PR is based on my personal motivation, I want to provide a working sample for it, but I cannot because my use case is based on separated proposal #9. When #13 (my PR for #9) is merged I'll try to provide some working sample here.

(I don't mind if it is merged by maintainers' decisions before I do it 👍)