Let the user choose how to install ltex-ls

[doums]

Hi,

It might be better not to make nvim-lsp-installer a mandatory dependency as it is, and make it optional.
I think there are a lot of people who don't use it (including me). And I just don't want to install it just to be able to use this plugin.
In the other hand, you could simply check if ltex-ls binary is present in the system (eg. using fn.executable()), if not fallback to ask the user to install it, either on its own or using nvim-lsp-installer through the command :GrammarInstall.

Currently, I have ltex-ls installed on my system, but I can't use the plugin, which is a stupid situation when we think about it.

#10