consider changing default segmentNameCharset to 'a-zA-Z0-9_'

[snd]

the current default is unexpected to some.

it's likely that this change will meet more expectations than the current default.

breaking change

see #32

[plandem]

same case :(

p.s.: also it's good idea to export definition of UrlPatternOptions

[snd]

the next release will be based on typescript and since it will contain breaking changes it will contain this change as well.

i've converted url-pattern from coffeescript to typescript over on this branch: https://github.com/snd/url-pattern/tree/coffeescript-to-typescript. tests are passing. just need to adapt the rest of the repo (build, CI, docs) to the new language.