Add g:ale_sh_shellcheck_change_directory

[fnichol]

This change adds a new g:ale_sh_shellcheck_change_directory option to control whether or not to change directory to where the source is located before running shellcheck. It defaults to 1, identical to the other linter directory-related options (I took heavy direction from the Python linters here) and so preserves the current behavior of ALE.

This is to help ShellCheck users that have a larger source base with sub-directories and CI that prefers to run from the root of a repository. As ShellCheck determines the relative directory in the shellcheck source=path/to/src.sh to be relative to the current working directory, it's hard to to have a value that satisfies both a *vim/ALE and a Makefile/TravisCI setup. The option here can be used as a first step to harmonize these two cases while any future features/changes in ShellCheck develop (see koalaman/shellcheck#769 and koalaman/shellcheck#539 for more context).

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Thanks so much for your hard work on ALE. I know I'm not alone, and I know maintainers don't hear this nearly often enough, but tools like this really make a huge difference in daily developer life. ALE has made a massive difference for me and how I work, so thank you! 🍺

[w0rp]

I finally had a chance to look at this, and it looks good to me. Cheers! 🍻