Added options to use custom font, variable pitch font, and custom font easily customizable font size.

[tefkah]

Hi! I've been using nano on and off, as while I really like the design philosophy, I really like using variable-pitch fonts while writing, which are unfortunately a little hard to use with nano as it does not play well with variable-pitch-mode or mixed-pitch-mode.
This PR is to "fix" that.

It does a couple of things, not all of which you may be a fan of as you mentioned in #18 , but I think at least some elements might be worthwile:

• Removes the hard-coded font definition from nano-layout
• Removes the hard-coded font definition from the nano-faces, instead delagating this to nano-default-face
• Adds a customization option for the font size, defaults to 14.
• Adds a customization option for the default font family. This still defaults to Roboto Mono, but is now customizable, with a warning that other fonts may not work.
• Adds nano-face-variable-pitch and the accompanying variable nano-family-variable-pitch, which, if set, sets variable-pitch to this face, allowing variable-pitch-mode to work properly.

All faces visible in this image. It works very well, have not yet encountered any problems.

Let me know what you think! I'm very happy that I can use nano with variable pitch now, I hope this might be useful to others who would like to as well. :) Thank you again for nano-emacs, it's really a delight.

[tefkah]

I have not yet tested this with all of the modules in nano-emacs, as I don't use mu4e and nano-writer does not jive well with my own org config, but I will try to test a few of them.

[rougier]

Thanks. I think it make sense to re-establish the variable pitch mode and I'll probably merge your PR.

I've just a few comments. Concerning the nano-font-size, what is the use exactly ? Because once you've started Emacs, this won't actually change the font size or will it?

Also, in the tag face definition, there's a font size "120" which corresponds to 85% of the default font size (rounded to nearest integer).

Also, for the font family names, maybe we could use nano-font-family-monospaced and nano-font-family-proportional

[tefkah]

nano-font-size wont change the font immediately, but if you call (nano-faces) (nano-theme) it will. Might be worth having an interactive function that does this, say "nano-refresh. as I find myself calling it quite often. I will include that in the docstring and create that function in the pr concerning #26!

Ah, I missed that definition, I'll change it.

Those sound better indeed, I've changed them.

[rougier]

Thank!