Add support for linters with non-zero exit codes and read from stderr

[gontzalm]

First of all, thanks for the awesome plugin.

I am trying to add the markdownlint-cli linter to guard.nvim. I encountered two issues:

• Currently guard.nvim does not support non-zero exit codes and markdownlint uses other exit codes.
• markdownlint outputs linter errors (diagnostics) to stderr, and guard.nvim discards this stream.
• markdownlint does not have an error severity, so the attributes.severity key cannot be specified in the lint.from_json function.

Are you planning to support linters with this behaviour? I attempted to fix this adding a valid_error_codes tool option and other changes in the commit gontzalm@ea99961.

Now the linter is working with the options

local ft = require("guard.filetype")
local lint = require("guard.lint")

ft("markdown"):lint({
    cmd = "markdownlint",
    args = { "--json" },
    fname = true,
    valid_exit_codes = { 0, 1 },
    source = "markdownlint",
    parse = lint.from_json({
        source = "markdownlint",
        attributes = {
            lnum = "lineNumber",
            col = "lineNumber",
            message = "ruleDescription",
            code = function(d) return d["ruleNames"][1] end,
            severity = function(_) return "warning" end,
        }
    })
})

require('guard').setup()

[xiaoshihou514]

Thanks for the attention! I am aware of this (#68). What I planned was separating some of the do_fmt and do_lint function into an api module. So ppl could write their own formatting/linting process when their tool it's behaving out of ordinary.
I feel this approach makes guard core less cramped (not having to parse an ever-growing tool specification) and enables users to implement stuff like #76 and #86.

I am (hopefully) going to work on this after the autocmd pr gets merged.

[xiaoshihou514]

Oh and from_json and from_regex is meant to be a template for the majority of tools so it's natural that it won't fit for some.

[imbolc]

Just found that Asqlfluff format too sometimes returns formatted text to stdout with a non-zero error code if there's still issues left.

[xiaoshihou514]

Just found that Asqlfluff format too sometimes returns formatted text to stdout with a non-zero error code if there's still issues left.

Yep, #150 should solve all of these :)
Though changes in guard-collection would be needed. I will also write some tutorials in ADVANCED.md