fix(issue #229): Concatenate all lsp client names found by GetLspClient

[winkee01]

As described in #229, this pull request fixes the issue. You can now use GetLspClient to retrieve all attached client names instead of only one of them.

An example of customization

local gls = require('galaxyline').section

table.insert(gls.left, {
    ShowLspClient = {
        provider = 'GetLspClient',
        highlight = {colors.textbg, colors.lspbg}
    }
})

Demo

[glepnir]

@winkee01 Please fix conflict