svg-lib-tag: redefine font-size with font-info val

It's unclear to the author of this commit why it makes difference,
however it fixes a problem on the Ubuntu system with IBM Plex Mono font.

See #14

charater -> character

Fix `font-info` query format.

From emacs info section, info:emacs#Fonts, font size should be prepended
with `-`, not `:`. The latter is to specify key-value pair.

This commit fixes a bug with font size specification per each SVG image creation.

Remove hard coded image scaling factor

Image scaling factor is governed by `image-scaling-factor` value, in
`create-image`. `svg-image` calls `create-image` under the hood, hence,
the user can restore scale factor of 1 by adjusting
`image-scaling-factor`.

Fixes #6.

Fix usage example in comment.

Fix string/array nil issue on windows

It saves to "~/.emacs.d/.cache/svg-lib/" by default. If `svg-lib-icons-dir`
does not exist, create it.

Icons are downloaded to `svg-lib-icons-dir` rather than cache by `url-cache`.

Cache by url-cache is not an ideal way for multiple platforms, since the
md5 of same icon on different platform is different.

This commit is downloading the icons to `svg-lib-icons-dir`.

This fixes the svg-lib-button/svg-lib-progress can not be used on
windows issue. Mentioned on issue #2.