Add prompt size limiting options

[fxdave]

• theme_prompt_max_size_percentage. When the prompt is wider then the given number, show a smaller prompt instead. The default is 90 (90% of the terminal's width). Fish cannot show wider prompt than 100% (this case fish would show: > ). See the option below.
• theme_prompt_keep_space. When the prompt is too wide to provide the given number of free space (chars), show a smaller prompt instead. The default is 10 chars. See the option above.