feat(#12): added :checkhealth freeze command

[AlejandroSuero]

Now we can :checkhealth or :checkhealth freeze when the plugin is loaded to get the information about dependencies for the plugin.

All required dependencies installed:

Missing required dependency freeze:

• checkhealth command
• autoload checkhealth
• plugin folder for requirements, it can be modified to alert if NeoVim version is lower than the APIs that we use, I think v0.8.0 is fine, but lower I think some are deprecated or not functioning.

[ethanholz]

Since this is checking for the existence of other tools (such as xclip), this is blocked by #7.

[AlejandroSuero]

@ethanholz it is that way in order to show that how we can add the necessary ones easily, but I can erase all except freeze, open and explorer if you want to merge it.