Adopt San Francisco / add font settings

[iccir]

Starting with 10.11, Apple is changing the system font to San Francisco instead of Helvetica Neue. The theme should be updated to use the new font, in order to match the system.

However, there are long standing issues with "Helvetica Neue" on Linux (See #8). My hunch is that this is due to the font having a space in the name, which may continue to be a problem with "SF UI Text". There may also be users that wish to continue using Helvetica Neue. Hence, I'd like to make the font a setting.

Based on @ig0r74 's fork, I'm tempted to remove the explicit "Helvetica Neue" font.face selectors. This will sadly use Lucida Grande on OS X unless http://sublimetext.userecho.com/topic/773897-/ is fixed. However, it should prevent issues on other platforms.

To force a specific font, you would set one of the following settings:

el_capitan_use_san_francisco to use "SF UI Text"
el_capitan_use_helvetica_neue to use "Helvetica Neue"
el_capitan_use_helvetica to use "Helvetica"

[iccir]

Updated settings:
el_capitan_font_default
el_capitan_font_san_francisco
el_capitan_font_helvetica_neue
el_capitan_font_helvetica

For now, the theme will ship with "Helvetica Neue" as the font. This will change to "SF UI Text" later this year.

Linux users affected by #8 can set el_capitan_font_default to true, which should be equivalent to @ig0r74 's fork.

[iccir]

Pushed to master. This bug can be closed once default font is "SF UI Text" (later this year).

[untone]

Please, add ability to change default font size in settings.

[iccir]

@untone - I'm not sure how to best do this, can you recommend other themes that have adjustable font sizes? Thanks!

[untone]

I mean ability to adjust sidebar font size.