Style support for status bar

[ivanmilov]

With this PR you can make statusbar output looks like e.g. powerline.
Example there: https://i.redd.it/13a2zz2ifjw31.png

New field in config: full_text_pattern
Value of this pattern will be parsed and all values surrounded by '%' will be used as parameter keys.
Values started with '&' - are references on another parameter keys.

Demo config example:

command=~/.config/i3/i3blocksScripts/$BLOCK_NAME
separator_block_width=0
markup=pango
color1=#ffffffa0
color2=#000000a0
background=#000000a0
separator_glyph=
full_text_pattern=<span foreground='%gfg%' background='%gbg%'>%separator_glyph%</span><span foreground='%fg%' background='%bg%'>%full_text%</span>
gbg=&fg
gfg=&bg

[foo]
command=echo foo
bg=&color1
fg=&color2
interval=once

[bar]
command=echo bar
bg=&color2
fg=&color1
interval=once

i3 config:

bar {
    position top
    i3bar_command i3bar -t 0.1
    colors{
        background #000000a0
    }

    status_command i3blocks -c demo.conf
}